Document installation of desimodel data files

[dkirkby]

The instructions here should be augmented with the specific steps needed to checkout the desimodel package from svn. Also, the desimodel link should point to the svn repo instead of the github repo.

[dkirkby]

I think this should work for any user (without special DESI privileges):

svn co https://desi.lbl.gov/svn/code/desimodel/trunk desimodel

Next, configure the DESIMODEL environment variable. This step is OS and shell specific, but for unix/mac and bash:

cd desimodel
export DESIMODEL=`pwd`

Please post corrections / improvements here.

[dkirkby]

@weaverba137 Any corrections or improvements to suggest for this documentation update?

[weaverba137]

I'm not sure why this is a specsim issue & not a desimodel issue. I think the install instructions in desimodel's top-level README.rst file are complete.

[dkirkby]

The reason is that someone starting with specsim wouldn't know to look at the desimodel docs, but that README has all the relevant info so I will just point to it.