0.7.41: fix: fix bugs in layoutelements (#393)

This PR fixes two bugs:

- fix a type casting issue when subtracting a int array with a float.
This popped up when testing with `unstructured`, and some sources of
element coordinates are of `int` type. This PR adds a new unit test case
for `int` coord type with the grouping function
- fix element class id 0 becomes None bug: this happens when dumping
`LayoutElements` as a list of `LayoutElement`. When an element class id
is 0 the logic on main would treat it as no existing and use `None` as
the type.