updates ier usage and prettifies code
adds line breaks to avoid warnings about too long lines
updates use statements
improved a few print statements and comments
added Ryan's lists of items and to-do list, and some answers from me
added a comment from Daniel Peter
Merge branch 'devel' of https://github.com/geodynamics/specfem3d into devel
deleted the redundant version of flexwin, which is stored as a stand-alone code at CIG/github (https://github.com/geodynamics/flexwin) and may be replaced with a new python version in the future
move ADJOINT_TOMOGRAPHY_TOOLS/ to utils/ to reflect the developmental (or obsolete) state of these tools; other efforts are ongoing outside of SPECFEM3D; see also src/tomography/
Update volumes.py

fix vol sandwich
Merge branch 'devel' of github.com:casarotti/specfem3d into devel
Merge branch 'devel' of https://github.com/geodynamics/specfem3d into devel
fix label for side block

this option fix a misalignment in the side block id (boundary_definition and cubit2specfem3d) 
the homogeneous_halfspace mesh generation was broken
Merge branch 'devel' of github.com:casarotti/specfem3d into devel
Merge branch 'devel' of https://github.com/geodynamics/specfem3d into devel
Undo comment