Omp

[lukeolson]

This start adding OpenMP support

• sparse.csr() is added to call an OpenMP SpMV in sparse.h when openmp=True is added to the .solve() phase
• The approach should work with and without OpenMP support
• setup.py takes the simple approach of attempting two different compiler flags.
• On testing:
  • a test on the SpMV in util/tests/test_sparse.py
  • builds with/without OpenMP should be added to CI

[codecov-commenter]

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[lukeolson]

Thanks @jbschroder.

I'm trying get set some consistent OpenMP flags with extension-helpers. Then I'll merge.

astropy/extension-helpers#40

[lukeolson]

getspams/spams-python#18