[Patch, Fortran] Add CO_REDUCE

[Tobias Burnus]

This patch adds some more checks and the actual implementation (compiler 
side and libcaf_single) for CO_REDUCE. It also rejects coindexed 
variables elsewhere, in line with the recent J3 changes. I also updated 
the API documentation (adding doc for collectives.)

Still unsupported as elsewhere are allocatable components and 
finalization – the latter also still has to be addressed in the standard 
itself.

Build and regtested on x86-64-gnu-linux.
OK for the trunk?

Tobias
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