Fix bug when subsetting mesh specified in another directory

Before, MPAS-Limited-Area would use the whole path name of the file specified
to create the subseted mesh, which would cause an error in its file creation.
This change uses os.path.basename which uses the Unix/Linux basename, which
returns the name after the last '/'.

Note: basename may not work if specifying other OS paths (e.g. Windows) on
Linux.

limited_area/limited_area.py

@@ -213,7 +213,7 @@ def create_partiton_fname(self, name, mesh, **kwargs):
     def create_regional_fname(self, regionName, meshFileName, **kwargs):
         """ Create the regional file name by prepending the regional name
         (specified by Name: ) in the points file, to the meshFileName. """
-        return regionName+'.'+meshFileName
+        return regionName+'.'+os.path.basename(meshFileName)
 
 
     # Mark_neighbors_search - Faster for smaller regions ??