Iterative inversion of deformation vector field with adaptive bi-residual feedback control

Updates:

• spatial domain prepadding in dvf.inversion to avoid boundary artifacts due to resizing in multi-scale iterations
• additional demo script: 3D DVF with spatially variant deformation along z-axis
• additional visualization of spatial maps for DVF inversion assessment in demos (IC residual magnitude fields, reference image recovery)
• some documentation extensions and clarifications

Bug fixes:

• dvf.feedbackControlVal: corrected locally optimal control values with 2D DVFs in the case of complex eignevalues
• dvf.inversion: removed problematic upper bound on neighborhood size for local control value search