Fix a couple type-conversion warnings.
Fix some set-but-unused variables.

In some cases, this involved removing the assignments. Others were
changing code to make use of the variables. Some cases I did not change,
since it appeared the unused assignments had value to give context to
other parts.
Fix an unnecessarily inverted condition.
Move force_ftz to the solver directory.

It's not used by xcreate_header_file or xmeshfem3D even though they link
with it.
Move make_gravity into specfem3D directory.

It's linked, but really unused by, the mesher. So it's really only
necessary for the solver.
Remove extra shared object from mesher compile.
Re-factor band_instrument_code to be clearer.
Remove euler_angles from specfem3d compile.
Remove memory_eval from specfem3D.
Remove unnecessary consistency check.

The code no longer uses the ceiling function, so there's no need to
verify that the result is correct. The code as it's now written will be
guaranteed to never fail this test.
Add a module wrapper so ifort doesn't complain.