Enhance PV to properly reflect snow coverage in hourly generation

[DavidGoldwasser]

Is your feature request related to a problem? Please describe.
Current REopt/PVWatts capability is to derate panels for a variety of factors like snow and dirt, but it is a single factor that impacts generation every hour our the day. While it may be able to reflect the annual generation isn't the most accurate way to represent generation hourly profile shapes.

Describe the solution you'd like
Better represent at each timestep the impact of snow on PV panels

Describe alternatives you've considered
I didn't see a way to do this in REopt or PV watts. I have an idea for how to doo this in EnergyPlus, but it depends on switch to using EnergyPlus PV described in issue #354

EnergyPlus shading surfaces support transmittance schedules, mean for something like deciduous trees that change seasonally or based on sun angle within a day. I propose to put an extra shading surface a few inches beyond the PV surface and use a schedule to reflect snow over on the impacts that reduce or eliminate generation.

Additional context
The more basic approach here is to expect a user defined schedule. A more advanced approach would make use of a combination of data in the weather file and the model to auto-generate reasonable schedule to start with. The EPW should have a field about snow cover on ground used to help inform the schedule. But rooftop snow may melt or slide off quicker than the ground, so some logic, maybe using sky condition and temperature, as well as surface tilt to estimate when they are cleared. Maybe this can be informed in future on measured generation data from a collection of buildings. The expectation is that steeper PV will clear of quicker than shallow PV angle.

While I see a pre-processor as easy approach, I suppose this could be down as EMS code and happen at each time step, if the right sensors and actuators exist. I know altering schedule value with EMS is possible.