Add t8_element_destroy statements to improve memory management

[lukasdreyer]

Describe your changes here:
Clean version of #676, closes #672
Memory that is allocated by t8_element_new, but not destroyed by t8_element_destroy, stays in the memory pool until the scheme is destroyed. This leads to an increasing memory footprint that doesn't show up as an error with the sc library or with valgrind.
Therefore, an assertion is introduced in the destructor of the scheme, that all elements were explicitely given back to the memory pool.
Additionally, many occurences of missing t8_element_destroy statements were found and fixed.

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[Davknapp]

Thanks @ackirby88 for finding this bug and your support in fixing it together with @lukasdreyer

It only needs some small adaptations and we are ready to merge!