Add t8_element_is_ghost function

[holke]

Describe your changes here:

Extend the t8_element_is_leaf functionality with t8_element_is_ghost and t8_element_is_leaf_or_ghost.
Given a ghost tree and an element this function returns true if and only if the element is a ghost element of that ghost tree.

Extended the test cases as well.

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[lukasdreyer]

I like your idea of seperating the functionality of choosing the element array and searching the element array.

But I think this approach can be done even stricter, by passing the element_array as argument to the helper function.

t8_forest_element_is_leaf and t8_forest_element_is_ghost then have the task to chose the correct element array, which they then pass to the helper function, which determines if the element is included in the element array.

[lukasdreyer]

Please resolve conflict and have a look at my comment regarding the separation of the helper functionality. I think we'd rather have the t8_element_array directly as input to the helper functionality