SynthPop compatibility updates

[hustonm]

Changes needed to make SPISEA compatible with the SynthPop Galactic modeling tool.

• Low mass objects below the isochrone grid can optionally be kept in clusters (off by default) and will have current_mass=initial_mass and phase=98, with no other evolutionary information or photometry.
• Evolution model versions are now logged in IsochronePhot files and checked if present.
• The option synthpop_extension is now available for MISTv1 version=1.2 evolution. This fills in the missing parameter space down to initial mass 0.1Msun in isochrones where needed. Use of this option will require downloading updated isochrone files.