Type annotations

[MichaelJFishman]

Description

Type annotations

Changed State to a typed NamedTuple

Split off from #67

Tests

Once #68 is merged, I'll rebase this branch and run the tests.
Ran unit tests.

[MichaelJFishman]

Some thoughts:

I think State should subclass gym.core.ObsType. There should possibly be other gym subclassing. Once the unittest PR is merged in, I may try that subclassing and see if it works. I'm not familiar with having a NamedTuple class that also subclasses another class, so I want to have the tests setup before making that change.

Some of the classes (eg Exists) could be cleaned up by using dataclasses, which can autogenerate __hash__, __eq__, __repr__, etc.

We may want to make Predicate, ForAll, etc. share a parent class, since all those seem like they'd be used in similar situations, eg as a goal.

I generally think that having stricter types for classes makes code easier to reason about, and enables more powerful static analysis. Currently PDDLDomain is initialized with most of its attributes as None. It would be nice if we could make PDDLDomain NEVER have None for most of these attributes, so that if you have a PDDLDomain object, you know for sure you can use it's actions, etc. I don't really know a good way to accomplish this without giving up the flexibility to parse a domain piecemeal. We could just make PDDLDomainParser prepare the attributes, then bundle them into a PDDLDomain that immediately has all the needed attributes. But I'm not confident this is the right move. Since I'm unclear about this part, I'm leaving that out of this PR for now.

[MichaelJFishman]

This branch passed the existing unit tests.