cross-tile spectra stack header cleanup

[sbailey]

This is a bookkeeping cleanup PR for desispec.spectra.stack for the case of stacking multiple tile-based spectra across different tiles. Previously this case generates a bunch of warnings like:

from desispec.io import read_spectra
from desispec.spectra import stack as stack_spectra
c1 = read_spectra('21/20210503/coadd-0-21-thru20210503.fits')
c2 = read_spectra('211/20210501/coadd-3-211-thru20210501.fits')
cx = stack_spectra([c1,c2])
...
WARNING: MergeConflictWarning: Cannot merge meta key 'TILEID' types <class 'int'> and <class 'int'>, choosing TILEID=211 [astropy.utils.metadata.merge]
WARNING: MergeConflictWarning: Cannot merge meta key 'TILERA' types <class 'float'> and <class 'float'>, choosing TILERA=217.828 [astropy.utils.metadata.merge]

and it incorrectly propagated per-tile keywords from the first spectra object. This PR detects the multi-tile case and removes the per-tile keywords if present before stacking, while also not modifying the input spectra.

I added unit tests to check the behavior (previously failing on main, now passing), and also tested on a tilenight+ztile run to make sure I didn't break the regular pipeline. This isn't strictly needed for Kibo, but I would like to get it in for the Kibo tag for convenience.