Cleanup C code slightly.
Remove duplicate constants.h file.
Correct licence in several files.
Remove todo file after moving issues to GitHub.
Remove commented code for absorbing boundary snapshots.

It was commented out almost 3 years ago; I don't think it's going to be
reinstated any time soon.
Remove redundant comment.

It's quite clear what the line does without the comment.
Remove irrelevant comments.

These are just code review; not actually explaining anything.
Remove code-duplicating comments.

These comments just duplicate some other code somewhere else. That's
just confusing, plus they're starting to get out of date now.
build: Remove Pyre checks.

The Pyrized version was removed in 2009, so these checks are not useful.
Fix some of the simpler warnings.
Use MPI_STATUS_IGNORE in MPI calls.

The status is not used, so this saves us an extra variable or two. As
suggested by @jedbrown in another PR for Specfem2D.
Move shakemap file to solver directory.

It's not really a shared file, as it isn't compiled into anything else.
Plus, it links against the solver static library.
Combine duplicate get_global subroutines.

These are all the same save for one loop bound. But that nested loop can
be re-written as a single loop that works for all cases.