Use instrument database for generating focalplane model

[tskisner]

This issue is to track the migration of the focalplane model generation from the current set of input files (fp_settings svn, DocDB files) to the online instrument database. This involves actually changing the desimodel.inputs.focalplane code to use this new upstream source of information, as well as adding more checks for consistency when loading info from that DB.

[sbailey]

FYI see the DESI wiki under Pipeline/RawDataTransfer#OperationsDatabase for information about how to access the read-only mirror of the ICS database at NERSC (and only visible from within NERSC, but I don't want to post the full info in a public ticket). Users can add those credentials to their .pgpass file at NERSC but we should not include that in the desimodel code itself (even if it is read-only and hidden behind NERSC credentials blah blah).

[sbailey]

Adding a note to prevent possible duplicated effort: @julienguy will take a first pass at this, updating the desi_generate_focalplane script to take a new parameter to load params from the database as an alternative to --pos_settings to get it from an SVN checkout of the files that were used to originally load the DB.

Possible gotcha: desi_generate_focalplane requires the configobj library, which isn't one of our standard dependencies and wasn't pre-installed on my laptop. "conda install configobj" easily fixed that, but it is highly likely that it isn't pre-installed in our latest desiconda release at NERSC either (but cori is still down so I can't immediately verify). If this is an issue ping me and we can discuss options.