Hardcoded plate scale model in obs_lsstSim

The plate-scale and pincushion distortion model used in obs_lsstSim to define the nominal on-sky geometry of the LSST focal plane is defined by two hard-coded numbers in obs_lsstSim/bin.src/makeLsstCameraRepository.py, camConfig.plateScale = 20.0 and pincushion = 0.925.

Doing it this way inhibits the use of the description-building script to experiment with alternative configurations, and obscures the existence of parameters that should be synced with the simulation just as the rest of the configuration in obs_lsstSim is supposed to be.

I suggest that these parameters be loaded from a new data file in obs_lsstSim/descriptions, and that a new argument be added to the above script to allow it to be loaded from an alternative location.

This could then enable the nominal model to be updated from the phoSim model as part of an extension of the procedure described in obs_lsstSim/Readme.md.

In addition, whether this ticket is rejected / deferred until the grand Obs-redesign or not, the source of these numbers should be documented.  Note that the SRD / KeyNumbers (https://www.lsst.org/scientists/keynumbers) value is 50.9 microns/arcsec.  Other sources have 50.0.  I have not been able to find a source for the pincushion value.