We're missing a lot of useful and sensible support for oneof fields

[CCP-Zeulix]

There are some cool features generated in the pb2 classes like "HasField()" and "WhichOneof()" to check which "oneof" field was/is used in the messages. The dc classes should really have something similar.

The pb2 classes also do stuff like clear all other fields in a "oneof" group when setting one fields. I'm not sure what would happen in Protoplasm in this case since we're going Dataclass -> dict -> PB2 Class so not really setting one field at a time. I'm guessing this will either raise an exception (preferred) or if there are internal loops that just iterate over the dict and set one field at a time, then which oneof field will be retained simply will be the "last" one iterated over, which will be very arbitrary in each case.