Added 3D mesh example

examples/dim3/homogeneous_halfspace/DATA/CMTSOLUTION

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+PDE  1999 01 01 00 00 00.00  67000 67000 -25000 4.2 4.2 homog_test
+event name:       homog_test
+time shift:       0.0000
+half duration:    5.0
+latorUTM:       67000.0
+longorUTM:      67000.0
+depth:          30.0
+Mrr:      -7.600000e+27
+Mtt:       7.700000e+27
+Mpp:      -2.000000e+26
+Mrt:      -2.500000e+28
+Mrp:       4.000000e+26
+Mtp:      -2.500000e+27

examples/dim3/homogeneous_halfspace/DATA/meshfem3D_files/Mesh_Par_file

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+#-----------------------------------------------------------
+#
+# Meshing input parameters
+#
+#-----------------------------------------------------------
+
+# coordinates of mesh block in latitude/longitude and depth in km
+LATITUDE_MIN                    = 0.0
+LATITUDE_MAX                    = 134000.0
+LONGITUDE_MIN                   = 0.0
+LONGITUDE_MAX                   = 134000.0
+DEPTH_BLOCK_KM                  = 60.d0
+UTM_PROJECTION_ZONE             = 11
+SUPPRESS_UTM_PROJECTION         = .true.
+
+# file that contains the interfaces of the model / mesh
+INTERFACES_FILE                 = interfaces3.txt
+
+# file that contains the cavity
+CAVITY_FILE                     = no_cavity.dat
+
+# number of elements at the surface along edges of the mesh at the surface
+# (must be 8 * multiple of NPROC below if mesh is not regular and contains mesh doublings)
+# (must be multiple of NPROC below if mesh is regular)
+NEX_XI                          = 36
+NEX_ETA                         = 36
+
+# number of MPI processors along xi and eta (can be different)
+NPROC_XI                        = 1
+NPROC_ETA                       = 1
+
+#-----------------------------------------------------------
+#
+# Doubling layers
+#
+#-----------------------------------------------------------
+
+# Regular/irregular mesh
+USE_REGULAR_MESH                = .true.
+# Only for irregular meshes, number of doubling layers and their position
+NDOUBLINGS                      = 0
+# NZ_DOUBLING_1 is the parameter to set up if there is only one doubling layer
+# (more doubling entries can be added if needed to match NDOUBLINGS value)
+NZ_DOUBLING_1                   = 40
+NZ_DOUBLING_2                   = 48
+
+#-----------------------------------------------------------
+#
+# Visualization
+#
+#-----------------------------------------------------------
+
+# create mesh files for visualisation or further checking
+CREATE_ABAQUS_FILES             = .false.
+CREATE_DX_FILES                 = .false.
+CREATE_VTK_FILES                = .true.
+
+# stores mesh files as cubit-exported files into directory MESH/ (for single process run)
+SAVE_MESH_AS_CUBIT              = .false.
+
+# path to store the databases files
+LOCAL_PATH                      = ./OUTPUT_FILES/DATABASES_MPI
+
+#-----------------------------------------------------------
+#
+# CPML
+#
+#-----------------------------------------------------------
+
+# CPML perfectly matched absorbing layers
+THICKNESS_OF_X_PML              = 12.3d0
+THICKNESS_OF_Y_PML              = 12.3d0
+THICKNESS_OF_Z_PML              = 12.3d0
+
+#-----------------------------------------------------------
+#
+# Domain materials
+#
+#-----------------------------------------------------------
+
+# number of materials
+NMATERIALS                      = 1
+# define the different materials in the model as:
+# #material_id  #rho  #vp  #vs  #Q_Kappa  #Q_mu  #anisotropy_flag  #domain_id
+#     Q_Kappa          : Q_Kappa attenuation quality factor
+#     Q_mu             : Q_mu attenuation quality factor
+#     anisotropy_flag  : 0 = no anisotropy / 1,2,... check the implementation in file aniso_model.f90
+#     domain_id        : 1 = acoustic / 2 = elastic
+1   2300.0   2800.0   1500.0   2444.4    300.0 0 2
+
+#-----------------------------------------------------------
+#
+# Domain regions
+#
+#-----------------------------------------------------------
+
+# number of regions
+NREGIONS                        = 1
+# define the different regions of the model as :
+#NEX_XI_BEGIN  #NEX_XI_END  #NEX_ETA_BEGIN  #NEX_ETA_END  #NZ_BEGIN #NZ_END  #material_id
+1             36             1              36        1        16              1

examples/dim3/homogeneous_halfspace/DATA/meshfem3D_files/interface1.txt

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+0
+0
+0
+0

examples/dim3/homogeneous_halfspace/DATA/meshfem3D_files/interfaces.txt

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+# number of interfaces
+ 1
+#
+# We describe each interface below, structured as a 2D-grid, with several parameters :
+# number of points along XI and ETA, minimal XI ETA coordinates
+# and spacing between points which must be constant.
+# Then the records contain the Z coordinates of the NXI x NETA points.
+#
+# interface number 1 (topography, top of the mesh)
+ .true. 2 2 0.0 0.0 1000.0 1000.0
+ interface1.txt
+#
+# for each layer, we give the number of spectral elements in the vertical direction
+#
+# layer number 1 (top layer)
+ 16