Feature ghost interface restructuring overlap

[antjeHP]

Closes #2235

Description
Another ghost definition has been added, called overlapping ghost.
Neighborhood is defined here as follows: Elements of a grid have a positive stretch factor. Elements can be stretched by this factor, overlapping (at more than single points or edges) with other stretched elements. If this happens, the elements are considered neighboring.
For example. If all elements have a stretch factor of 1.0 or less, no element has a neighbor.
Another example: Given a uniform square grid all with a factor of 1.3, the neighbors by overlap correspond to the neighbors by common corners.

How works this definition:
The class of the overlapping ghost definition inherits from the base class for ghost definitions. The procedure of the do_ghost method is therefore similar.
A definitions object can be created either with or without a uniform stretch factor for all elements. However, a uniform factor can also be added or removed later.
Since the entire process of searching and exchanging the ghost element is encapsulated in the do_ghost method, the uniform factor cannot be changed instead.

The procedure of the do_ghost method:

• The procedure of the do_ghost method is similar to that of the base class. First, the communicate_ownerships method of the base class is called. If there is no uniform stretch factor, the maximum stretch factor of each process is determined and exchanged.
• Subsequently, each process creates a so-called cover locally for all processes, based on the first leaf element of each process previously transmitted. A set of elements is a cover of a process, if for each leaf element in the process, there is an element in the cover, that is equal to an ancestor of this leaf element or equal as the element it self. The elements of the cover are either given the uniform stretch factor or the maximum of the associated process.
• Now the actual search for the ghost elements begins. Each process searches its elements for those that are neighbors of other processes in order to send these elements to this process. The neighborhood is determined by the local cover of the other process and not by the actual elements of the other process, as one process does not even know these elements locally.
• In the last step, the communicate_ghost_elements of the base class is called and then clean up: only the memory required for the search is released again.

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