Bugfix: fewer obs than mpi processes

[Toby Searle (twsearle)]

Description

Ensure that MPI communication happens where all processes will execute it. This prevents deadlock when one or more processes have no observations and so never attempted MPISync:

nemo-feedback/src/nemo-feedback/NemoFeedback.cc

Line 511 in d665396

std::tuple<util::DateTime, size_t> NemoFeedback::mpiSync(size_t nLevelsLocal)

Issue(s) addressed

Resolves #76

Impact

Eliminate operational failures when very small number of obs pass QC.

Testing

• exab_gulf configuration
• exab_amm7 configuration
• exab_amm15 configuration
• exab_ostia configuration
• exab_ocnd configuration
• exab_gl_ocn configuration

Checklist

• I have updated the unit tests to cover the change
• New functions are documented briefly via Doxygen comments in the code
• I have linted my code using cpplint
• I have run the unit tests
• I have run mo-bundle to check integration with the rest of JEDI and run the unit tests under all environments

[Simon Good (s-good)]

This looks great. My only comment is unrelated to the changes, and is just about an apparently redundant line of code that looks like it could be deleted.