The big dadaflow merge

Dadaflow is now separated from the rest of the cmake build in its own submodule. This is not enabled by BUILD_SUBMODULES but by a new option BUILD_DADAFLOW (defaulted to OFF).

A couple of concerns here:

• the inclusion of the mathdx dependency is not well handled at the moment.
• this changes the C++ language standard in cmake
• Additional warning have been enabled. The merge request includes some fixes to non-dadaflow parts of psrdada_cpp

added Feature label

requested review from @twinchen

assigned to @ebarr

On the mathdx side I note that Bram over in Astron has written this: https://git.astron.nl/RD/mathdx

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• 6116b291 - added base class for resource allocator and sigproc unpacker tests

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• eecbfed3 - added docstrings

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• b940e3f8 - removed typo

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• 2796d7fc - added explicit header set specialisation for strings

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• 4e2de1eb - updated to format epoch and picoseconds correctly for DSPSR

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• afced011 - Remove fftdetacc from dadaflow and its mathdx dependency

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merged

mentioned in commit ca9b4c9a