Pourbaix: default to filter_solids=True

[rkingsbury]

Summary

Default PourbaixDiagram to use filter_solids=True for consistency with the MP website and discussion in #2173.

In addition:

• Fix a bug in MaterialsProjectAqueousCompatibility introduced by e8028b0
• add type hinting to PourbaixDiagram and make a few type hinting fixes to Entry classes (necessary to make mypy happy on PourbaixDiagram).
• Deprecate the include_unprocessed_entries kwarg to PourbaixDiagram.as_dict. This was necessary in order to make serialization work properly after switching to filter_solids=True by default. To me, it appears this kwarg was a sort of workaround to allow a user to effectively decide whether to filter solids at the serialization / deserialization stage. In my mind, it makes more sense for the user to make that decision via the filter_solids kwarg when the diagram is constructed, then serialize the filter_solids kwarg along with all unprocessed entries always. I feel this is more consistent with the way I "expect" serialization to work. @montoyjh please comment if you have any concerns or had a specific reason for keeping this is a kwarg to as_dict

Closes #2173

[montoyjh]

I think it was more intended to include single entries that one might want to inspect and use for analysis post serial/deserialization in the multi-entry case, but I think it's probably overly complex and misnamed, so I'm in support of getting rid of it.

[mkhorton]

Thanks @rkingsbury, @shyamd, including discussion in e8028b0#commitcomment-52383083