Change the elements_are_familiy arguments

[Davknapp]

I think we missed a const in the elements_are_family interface.
The pointer to the element itself are not modifyable, however the elements itself are.

changing it to
elements_are_family (const t8_eclass_t tree_class, const t8_element_t *const *fam)

should allow us to pass a family of const elements to it. That way if we gather elements via a call from
t8_element_array_index_locidx_t we can directly pass them to the function.

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