Rethink Dtype class hierarchy

[MJJoyce]

We should rethink the way we're structuring our classes in dtype.py. Currently we have everything inheriting from Primitive type and it's turned into a bit of a messy catchall for all sorts of stuff. The inheritance of Complex types from it ends up being, at best, meaningless since the vast majority overwrite the encode / decode methods. Things like Time40 ends up shoehorned in by setting a 4-byte primitive type and then manually overwriting the byte/bit counts to support the 5-byte type.

TLDR: PrimitiveType isn't great.

I think we'd be better served by throwing an abstract base class in there to mandate the API that we require. It would likely make the user-facing portion of it more coherent as well.