feat(model): Add input/output_exchanges to fa.Function

[ewuerger]

To be on par with inputs/outputs on oa.OperationalActivitys these
two properties were added.

[Wuestengecko]

We keep adding more and more special cases like this. I think we should rather look at how we could take a more generalized approach, to make it easy for users to express the exact relationships they're interested in.

I see a case to make some generalized filtering and mapping methods on ElementList, which would transform the list by accessing the given attribute on each item. This example could then be written similar to:

my_func = model.search("LogicalFunction")[0]
exc = my_func.inputs._the_mapper_function_("exchanges")
# exc now contains the same as this PR's `my_func.input_exchanges`

Of course the downside is that this cannot be statically type annotated (to the best of my knowledge), so the static type of exc would simply be ElementList[Any].

This is just a rough idea of course, but what do you think? Does it make sense to explore this idea further?