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 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confusing, plus they're starting to get out of date now.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.