default DESI_SPECTRO_REDUX

[sbailey]

This PR makes $DESI_SPECTRO_REDUX optional for the purposes of desispec.io.findfile. If it is set, it will be used just like it currently is, but if not set it will default to $DESI_ROOT/spectro/redux. I updated the tests to confirm this behavior. The motivation for this is to make the required environment setup for end-users as simple as possible, in particular for the tutorials ($DESI_ROOT will be the only required environment variable for end-user analysis, while still allowing $DESI_SPECTRO_REDUX override). Ditto for $DESI_SPECTRO_DATA defaulting to $DESI_ROOT/spectro/data if not set.

I did not scrub $DESI_SPECTRO_REDUX from absolutely everywhere in desispec, so I suggest that we continue to set it and use it like before. I did update two custom path creations with their equivalent findfile invocations.

[coveralls]

coverage: 38.991% (+0.002%) from 38.989%
when pulling b2893af on default_redux
into 0852688 on main.

[akremin]

Thanks, the updates look good and bonus points for the changes.rst update and unit tests. I have one quick request inline that should be trivial to change.