Handling of zero input parameters

Several phasor dynamics models currently use != 0, == 0, != ZERO<RealT> or == ZERO<RealT> for some input parameters to enable/disable some features. This is ill advised for floating point values.

The models currently affected are:

• ESDC1A
• HYGOV
• IEEEST
• IEEET1
• REECB
• REPCA

isEqual with a tolerance or splitting == into isfinite(), <= and >= are inadequate here. The desired behavior really is to check if the value is zero, and the underlying purpose is to enable/disable some options.

One proposed solution is to expose the enabled/disabled options directly as integers (e.g., order for a stabilizer or booleans (e.g. saturation_disabled for an exciter). While this duplicates some information in the input file, this should make the model implementation cleaner. As long as the enabled/disabled features don't change during the simulation, it can be argued that we are mathematically dealing with different models.

cc @pelesh @superwhiskers