Upgrade gstlal_postcoh_skymap2fits to Python 3

Merged Patrick Clearwater requested to merge pwc_p3_sm2fits into master

Python 3 version of gstlal_postcoh_skymap2fits.

To test, run pipe.sh or equivalent, and then:

gstlal_postcoh_skymap2fits --output-cohsnr ptr_cohsnr2.fits --output-prob ptr_prob2.fits --event-time 1187008882 --cuda-postcoh-detrsp-fname [wherever it is] --event-id GW170817 H1L1V1_skymap/L1_1187008882_445800781_3_806
ligo-skymap-plot -o output-snr-viridis_ptr.png --colorbar ptr_cohsnr2.fits  --colormap viridis
ligo-skymap-plot -o output-prob-viridis_ptr.png --colorbar ptr_prob2.fits  --colormap viridis

Thankfully this has no dependencies that won't run in Python 3, except for pipemodules. See hacky changes in the diff - they don't seem to have caused problems though.

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