Adds feature SPECFEM3D_GLOBE mask source in kernels

[bch0w]

SPECFEM3D_GLOBE has the ability to generate gaussian source masks which are meant to suppress the high amplitude gradient values centered around the seismic source. In standard operating mode, these are output as binary files. This PR adds the ability for SeisFlows to read in, modify, and apply these source masks to event kernels automatically. It also adds some additional riders including features and bugfixes.

Changelog:

• New Solver parameter mask_source toggles on/off SPECFEM source mask generation
• New Solver parameter scale_mask_region allows modulating the amplitude of the source mask. This is useful, e.g., if you want to fully cut out the source region, or if the amplitude of the kernels is too small for the default source mask to effectively suppress the kernel values
• Migration workflow applies source mask by reading it in with the Model class and applying it to all available kernels prior to kernel summation
• Applicable to all workflows that derive from Migration (Inversion, NoiseInversion)

Riders:

• Feature: Fixed up Solver.combine_vol_data_vtk for easier VTK file generation from inside debug mode
• Bugfix: fixed updated namespace change for CL tool seisflows swap
• Bugfix: seisflows configure was writing default paths twice in the parameter file
• Aesthetic fix: cleaned up log messages associated with Solver directory intialization
• Aesthetic fix: removed unnused parameter export_vtk from solver modules
• Aesthetic fix: all base class docstrings now indicate that they are a base class for improved parameter file docstring
• Aesthetic fix: added version number to base parameter file for more consistency across version updates