Optional addition of ocean_bgc into mom6 libraries

[liuxiao37k]

Modified CMakeLists.txt to add optional build of ocean biogeochemistry (ocean_bgc) source code into mom6 libraries/executables, invoked by {$OCEAN_BGC_FOUND} = 1

Modified README.md accordingly.

These updates will not affect the existing soca-mom6 ctests (i.e. they can still run with mom6 physics only without ocean_bgc) - they only built the capacity for running ocean_bgc.

Issue(s) addressed

• fixes JCSDA/soca-bundle/issues/25

Dependencies:

An upstream repository needs to be merged first:

soca-bundle: branch = feature/ocean_bgc
https://github.com/JCSDA/soca-bundle/tree/feature/ocean_bgc

waiting on JCSDA/soca-bundle/pull/26

[codecov]

Codecov Report

Merging #10 into dev/master-ecbuild will not change coverage.
The diff coverage is n/a.

@@                 Coverage Diff                 @@
##           dev/master-ecbuild      #10   +/-   ##
===================================================
  Coverage               43.20%   43.20%           
===================================================
  Files                     213      213           
  Lines                   62214    62214           
===================================================
  Hits                    26880    26880           
  Misses                  35334    35334           

Impacted Files	Coverage Δ
src/framework/MOM_horizontal_regridding.F90	0.00% <ø> (ø)

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[liuxiao37k]

Could you please give me a hint on how to get the following done (as you commented here: https://github.com/JCSDA/soca-bundle/pull/26)? Thank you! @travissluka

"I think the ocean_bgc CMakeLIsts.txt needs to

export the files to be included in MOM6. It's messy having the list of variables duplicated in the MOM6 CMakeLists here https://github.com/JCSDA/MOM6/blob/feature/mom6-bgc/CMakeLists.txt#L130 AND this wont work when doing TravisCI builds of soca because those locations can't be hardcoded like that. Their location will be different when doing a make install.
move the recursive submodule update to inside the ocean_bgc repo, That's too much logic to be in the main bundle right now.
Perhaps we should have a discussion on the particularities of how our TravisCI build system works, because it has implications on how new repositories are added."

This is what I tried but it did not work:

in ocean_bgc/CMakeLists.txt:
set (OCEAN_BGC_SRC_FILES ocean_bgc_src_files)
list( APPEND ocean_bgc_src_files
file aaa
file bbb
file ccc
...)
in mom6/CMakeLists.txt:
ecbuild_use_package( PROJECT ocean_bgc )
list( APPEND mom6_src_files
${config_src_files}
${src_files}
${OCEAN_BGC_SRC_FILES}
)
How could I export OCEAN_BGC_SRC_FILES such that it is accessible in mom6? Thank you.

[guillaumevernieres]

@liuxiao37k , fyi, @aerorahul 's recursive cloning addition to ecbuild is curently in review.