Cleanup use of `lark.Tree` vs. `Expression`

Pick one and use it consistently. See `celpy/__init__.py` for the definition of the `Expression` type. If this is to be used throughout, move it to `celparser`,

Start by reading the Expression definition in celpy/__init__.py and locating uses of both lark.Tree and Expression. Determine whether Expression should move to celparser, then make the representation consistent throughout the affected code. Done means the project uses one chosen type consistently.

Written by the indexing model from the issue text.