add support for processing a custom sample with full MPI

[moustakas]

This PR adds hooks which allows one to pass a custom / input sample to mpi-fastspecfit. Currently, only healpix coadds are supported and one must pass an input --samplefile argument which provides the full path to a FITS catalog which contains (at minimum) SURVEY, PROGRAM, HEALPIX and TARGETID (all other columns are ignored).

I've used this wrapper successfully on one of @dirkscholte's projects of ~200k objects (spread across more than 14,000 healpix coadds in Iron, but with typically only 30-75 targetids per coadd), but will test on one other small sample (a sample of ~70 spectroscopic lenses in Fuji) before merging.

[coveralls]

Pull Request Test Coverage Report for Build 7816323936

• 3 of 7 (42.86%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 59.006%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
py/fastspecfit/io.py	3	7	42.86%
<!--	Total:	3	7	42.86%	-->

Totals
Change from base Build 7668766351:	0.005%
Covered Lines:	2506
Relevant Lines:	4247

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💛 - Coveralls

[moustakas]

Coverage test failure is due to the intermittent (and annoying) Could not submit coverage: 422 Client Error; merging.