Enhance CoilHeatingGas model.

Add support for CoilHeatingGas::partLoadFractionCorrelationCurve.
Enhance SDD translator.

Added support for several more HVAC properties in SDD translator.
Merge remote-tracking branch 'origin/develop' into sdd_enhancements
Enhance SDD translator

Add support for still more HVAC properties in SDD translator.
Specifically add support for SDD defined backup heating coil inside of PTHP instead of
using a default electric coil.  Add support for PTHP coil lockout
properties.
Merge pull request #444 from NREL/sdd_enhancements

Sdd enhancements