account for major pvwatts changes

The SAM Version 2021.12.02 release notes describe major changes to the PVWatts model:

With this release, we have updated SAM’s implementation to PVWatts V8 (pvwattsv8 in SSC) with new features listed below. The online PVWatts® Calculator continues to use pvwattsv5 and will be updated later. All versions of PVWatts are available via the SAM Software Development Kit and PySAM. (729)

Reorganize user interface to group advanced inputs into a collapsible panel.
Add snow loss model when weather file includes snow depth data in cm.
Support for bifacial modules.
Internal module and module thermal models based on same model as Detailed PV model.
Internal inverter model based on same model as Detailed PV model.
Inputs for monthly soiling as DC loss.

We should add a note or a warning to pvsystem.pvwatts_dc and inverter.pvwatts. And ModelChain.with_pvwatts is also now ambiguous.

[adriesse]

The following note from the SAM website may also be relevant:

Note: The PVWatts website uses a different version numbering system than the PVWatts model. For example, the website “Version 6” uses the PVWatts Version 5 model.

[cwhanse]

After thumbing up, second thought: append v5 to base layer functions but add a parameter to ModelChain.with_pvwatts.