Add an isWater method for Groups

Adds an `isWater()` method to the `Group` interface, which is implemented in `HetatomImpl`; `ChainImpl`'s static field `waternames` has been moved to `GroupType.WATERNAMES`.

Updates `StructureTools`, `TestAltLocs`, and `ChainImpl` to use the renamed and relocated field.

Note that this technically this commit would break backward compatibility; if someone relied on using `ChainImpl.waternames` directly, then their code would no longer work.