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This commit contains a number of major changes and removals of unused source which aims at slimming down the source code in preparation for a release.

Switched to using CommData_t by value instead of by reference. MPI_Comms now freed via an std::set.

Got rid of the deprecated subdirectory.

Got rid of inner mapping functionalities which were not used due to being inefficient.

(somewhat questionable decision) got rid of old unit tests contained in unit_test subdirectory, including the code needed for the VERIFY functionality used in dist_tensor_op.cxx.
The old unit tests were rather limited and at this point somewhat redundant, but there may be reason to use VERIFY functionality in the future.