Don't downcast molecules to single residues and atoms. [closes #16]

[lohedges]

This pull request fixes issue #16

With the Sire search updates for the 2023 release we now have .molecules(), .residues(), etc methods on the search result to cast to the required object type. However, we still retained the existing code that would (potentially) downcast object types on extraction with __getitem__. For example, single residue molecules were converted to residues, single atom molecules were converted to atoms, etc. Now we want the user to do this directly by calling the appropriate method, e.g. .residues(). This oversight caused issues in the isSame method of BioSimSpace._SireWrappers.System, where non-water molecules were compared based on their properties. If a molecule only had a single residue, then it would be returned as a residue and .propertyKeys() would return an empty list, rather than the required molecular properties. The result was that systems were compared as equal, when in reality they weren't, e.g. different coordinates.

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Suggested reviewers:

@chryswoods

[chryswoods]

That looks good and makes sense for the way search is used in BioSimSpace :-)