Kill OpenMP parallelism in gravmag forward modeling

[leouieda]

This causes so many install problems that it is not worth the small speed gain (see #164). Resulted in #165 to replace Cython OMP by Python multiprocessing.

This PR removes the prange from the Cython modules and stops linking omp to the compiled extensions. The install docs no longer need to mention installing TDM-Gcc in Windows.

A later PR will implement the higher level parallelism, probably simultaneously with the Cython/numba conversion (#169).

Checklist:

• Make tests for new code
• Create/update docstrings
• Include relevant equations and citations in docstrings
• Code follows PEP8 style conventions
• Code and docs have been spellchecked
• Include new dependencies in docs, requirements.txt, README, and .travis.yml
• Documentation builds properly
• All tests pass
• Can be merged
• Changelog entry

[leouieda]

This solution seems to have worked for users having problems on MacOS (as this StackOverflow question).