Add pfsSimObject module

We need a pfsSimObject module. Proposed changes from existing datamodel.txt are currently only:

1. Definitely add a LINETBL HDU, with at least lambda, amplitude, width, name columns.
2. Possibly drop FLUX image, since the reference data are in the FLUXTBL HDU. But FLUX could be useful for quick identification or for some users. In any case, assert that FLUXTBL is definitive.