Promote CHGNET and M3GNET to full dependencies

[ElliottKasoar]

Resolves #161

• Moves chgnet and m3gnet dependencies from optional to full dependencies, and removes pytest decorators, which prevent related tests running by default.
• Adds singlepoint tests for both

Note: Currently requires pinning dgl, as attempting to install the most recent version gives me an error: ERROR: Could not find a version that satisfies the requirement dgl==2.2.1 (from versions: 0.1.0, 0.1.2, 0.1.3, 1.0.0, 1.0.1, 1.0.4, 1.1.0, 1.1.1, 1.1.2, 1.1.3, 2.1.0) (see issue linked in #161).

[ElliottKasoar]

The README probably could do with updating too to reflect this, including a comment about setting the PyTorch dtype if using multiple MLIPs. (Ideally I'd like to implement a solution where the default dtype is set on use of the calculator in some way, so it's hidden from the user completely, but that's non-trivial.)