Feature: Ghost interface with t8code module structure

[sandro-elsweijer]

Closes #1784

Describe your changes here:
Separates ghost algorithm into its own abstract class. The structure is similar to the geometry module, but there is no ghost handler yet. But it should be easily implementable if needed in the future.
original work from @antjeHP.

There now is a ghost base class, which can be derived from to create a new ghost definition.
There is also a class which implements a base for search-based ghost algorithms and a class which implements our normal face ghost algorithms.
I tried to separate the functions needed by the actual ghost class into t8_ghost_definition_helpers and the rest (ghost element access and information, remote information etc.) stayed in t8_forest_ghost.

I am sorry, but I was not able to break the PR down into smaller bits. Most of the code just switched files, only the actual interface and ghost classes are new.

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