Minor PR: add option bin/desi_average_flux_calibration

[julienguy]

This is really a tiny difference that does not affect the pipeline:
Add option to desi_average_flux_calibration to remove flatfield correction.

[sbailey]

Looks good, but I made two small changes:

• use desispec.io.util.replace_prefix instead of filename.replace(...). Simply using replace breaks if you have a specprod with a name like "test-fluxcalib" (we've encountered variants of this problem multiple times with other filename manipulation, thus the existence of replace_prefix to replace only the filename prefix and not any other places where it appears in the path or filename)
• print -> log.info

Since I don't know example inputs to test this, please retry this branch with your work to make sure I didn't introduce a typo, then ok to merge.

[julienguy]

I checked your change. Thanks.