modify build vars through coupling files

[mandresm]

While trying to get MPIESM to work I realized that the following lines in the mpiesm.yaml were not making it into the conf_command of MPIOM:

mpiesm.yaml

general:
        [ ... ]
mpiom:
        conf_command: conf_command: ./configure --prefix=${model_dir} --disable-mh-file --with-coupler=oasis3-mct --enable-HAMOCC --with-mpi=intelmpi

Instead, the conf_command from mpiom.yaml was used, which has no --with-coupler=oasis3-mct. Therefore, a simulation with MPIESM was not possible as it was compiling MPIOM as standalone.

The problem was that no variables from the component sections of the setups are used by esm_master, but this is meant to be like that by design. The solution here allows you to introduce build-changes for coupled setups through the coupling files, instead.

Now it is possible to define variables inside the coupling files, under a dictionary coupled_component_changes, so that changes in the building variables can be performed.

This is how the coupling file mpiom-1.6.3p2+echam-6.3.05p2 for MPIESM looks like now:

components:
- echam-6.3.05p2
- mpiom-1.6.3p2
- oasis3mct-2.8
coupling_changes:
- sed -i '/ECHAM6_COUPLED/s/OFF/ON/g' echam-6.3.05p2/CMakeLists.txt
- sed -i '/(OASIS3MCTROOT)/s:include:oasis/build/lib/psmile:g' mpiom-1.6.3p2/configure
coupled_component_changes:
    mpiom:
        conf_command: ./configure --prefix=${model_dir} --disable-mh-file --with-coupler=oasis3-mct --enable-HAMOCC

Note: It only adds these variables if they already exist in reduced_config. This is done this way to limit the variables that can be defined in the coupled_component_changes to only the ones in relevant_entries (the ones necessary for compilation). Remember, so far the couplings files are only build-related, runtime configurations should go into the setups files.

[mandresm]

There was already a possibility to do this through compiletime_environment_changes, so adding a new functionality is not worth it. Therefore, I close this pull request.