updated BUNIT to proper units

[rstaten]

This PR:

• fixes cframe files should set BUNIT keyword #283
• adds 1e-17 to the BUNIT keyword

[sbailey]

Thanks! This PR fixes the BUNIT for fluxcalib output. Could you also add BUNIT for cframe? In that case I think it is completely missing.

[rstaten]

It is completely missing, but the cframe files seem to be generated using the same write_frame function as the frame files. Since these two files do not have the same output flux units, this may be why the BUNIT keyword is not included. If desired, I could simply write a new function or change the current one to distinguish between the outputs.

Also, I'm assuming the stdstars output should also include the factor of 1e-17 as well? If so, this is a simple fix.

[sbailey]

It is completely missing, but the cframe files seem to be generated using the same write_frame function as the frame files. Since these two files do not have the same output flux units, this may be why the BUNIT keyword is not included. If desired, I could simply write a new function or change the current one to distinguish between the outputs.

Good point. Let's add an optional units keyword to desispec.io.write_frame, with something like

if units is not None and 'BUNIT' not in hdr:
    hdr['BUNIT'] = str(units)

That should work for units as a string and if units is an astropy.units.Units object. To check: are there any python 3 unicode gotchas? I hope not...

And then update the code that calls desispec.io.write_frame with the proper units for frame vs. cframe.

Also, I'm assuming the stdstars output should also include the factor of 1e-17 as well? If so, this is a simple fix.

Yep.

[rstaten]

I have set the BUNIT for frame files to photons/bin and cframe files to 1e-17 erg/s/cm^2/A.

[sbailey]

We had a misunderstanding about my suggestion for how to propagate units to the frame files. Implementing it was easier than re-describing it, so I went ahead and changed it. Since we do propagate header keywords from inputs to outputs, I would like to run an integration test on this to check if we end up with any intermediate steps with incorrect BUNITs. i.e. don't merge yet; I'll fix or report any problems, then merge.

[sbailey]

I made a number of small changes:

• Added more BUNIT throughout
• Made BUNIT strings that can be parsed by astropy.unit.Unit (even if we aren't doing that in the main code). e.g. "Angstrom" instead of "A" and "electron" instead of "electrons".
• fixed fiberflat single- vs. double-precision when reading in (unrelated to original purpose of this PR)
• fixed fiberflat EXTNAME (now "FIBERFLAT" not "FLUX"; unrelated to original purpose of this PR)

I checked this with an integration test to make sure that BUNITs weren't incorrectly propagated from one file type to another. After the Travis tests pass I think this is ready to merge. Thanks @rstaten for getting this started.