Some tutorials hard code paths

[akremin]

Some tutorials use hard-coded pathnames that only work at NERSC. It would be ideal if we could use the standard environment variables DESI_ROOT, DESI_SPECTRO_REDUX, SPECPROD, etc. to allow users to set up their own environment on their own system without needing to edit lines of code. This is also relevant for the desidocker work, which gets around this with setting up fake directory trees, but it would be ideal if we didn't need to have such workarounds.

E.g. tutorials/getting_started/intro_to_DESI_EDR_files.ipynb hardcodes the following:

specprod = 'fuji'    # Internal name for the EDR
specprod_dir = '/global/cfs/cdirs/desi/public/edr/spectro/redux/fuji/'

But it could check for environment variables first, and only default to the above if the variables aren't set.

[sbailey]

In the tutorial updates for DR1 in PR #110 I updated all of the getting started tutorials to be based upon $DESI_ROOT without hardcoding NERSC-specific paths, except for the QuickStart tutorial where I included a NERSC-specific fallback if $DESI_ROOT was not set but $NERSC_HOST was set. These were re-organized and developed on my laptop with the wifi turned off to confirm that they work away from NERSC. I did not retroactively update past meeting tutorials, etc., but I think that is good enough for the spirit of this ticket.

Let's hold ourself to this standard for future 01_getting_started and 02_digging_deeper tutorials — even if they require running at NERSC, it can be useful to swap $DESI_ROOT to other data releases without hardcoding specific NERSC paths.