Introduce interface function t8_element_equal

[lukasdreyer]

Describe your changes here:
Introduce new interface function t8_element_equal, that only compares its contents and not the order in the scheme.

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[Davknapp]

Looks good.
However, the function itself is not tested. Can you add a test that checks if equality and inequality of elements is detected correctly?
(create a uniform mesh, two nested loops, check equality if the iterators are equal, inequality else)

After discussion with @lukasdreyer we postponed it to make a general dfs-style test