avoid TTYPE etc. FITS warnings when writing frames with fibermaps

[sbailey]

An unintended side effect of PR #1454 to support fibermap reading with astropy 4.3.1 (i.e. by using fitsio instead and then putting into an astropy Table), was a bunch of warning messages when trying to write it back out as part of desispec.io.frame.write_frame due to it using astropy.io.fits.convenience.table_to_hdu:

Current master produces 178 warning messages per call to table_to_hdu (which gets multiplied by 30 when wrapped by the pipeline for 30 cameras in parallel...)

>>> import desispec.io
>>> from astropy.io.fits.convenience import table_to_hdu
>>> fm = desispec.io.read_fibermap('fibermap.fits')
INFO:fibermap.py:335:read_fibermap: iotime 0.081 sec to read fibermap.fits at 2021-11-08T21:06:39.209923
>>> table_to_hdu(fm)
WARNING: Meta-data keyword XTENSION will be ignored since it conflicts with a FITS reserved keyword [astropy.io.fits.convenience]
WARNING: Meta-data keyword BITPIX will be ignored since it conflicts with a FITS reserved keyword [astropy.io.fits.convenience]
...
WARNING: Meta-data keyword TTYPE70 will be ignored since it conflicts with a FITS reserved keyword [astropy.io.fits.convenience]
WARNING: Meta-data keyword TFORM70 will be ignored since it conflicts with a FITS reserved keyword [astropy.io.fits.convenience]

This PR uses desispec.io.util.addkeys in desispec.io.fibermap.read_fibermap to not put these keys into the fibermap.meta dictionary in the first place:

>>> import desispec.io
>>> from astropy.io.fits.convenience import table_to_hdu
>>> fm = desispec.io.read_fibermap('fibermap.fits')
INFO:fibermap.py:335:read_fibermap: iotime 0.077 sec to read fibermap.fits at 2021-11-08T21:11:48.806477
>>> table_to_hdu(fm)
<astropy.io.fits.hdu.table.BinTableHDU object at 0x2aaaaac312e0>

Note: this isn't a complete solution because the comments associated with each header keyword still get lost, but they were getting lost before anyway. PR #1459 is fixing that when creating the original fibermap, but we'll need more work downstream to make full use of that. This PR just just trying to quiet down the 5k warning messages when writing frames.

[coveralls]

Coverage remained the same at 26.274% when pulling 507d956 on fibermapio into bee10be on master.