Add v4.2.0, generalise COMPONENTS (i.e. OPA/TOP) and CPP_KEYS

[jpolton]

Done: Edits to add 4.2.0 via gitlab repo.

WIP: add multiple words to variable definitions in NEMO_in that can be passed to setup_nemo
E.g. this would be ideal

MY_SRC=                        # If blank no need to do anything
NEMO_VERSION=4.2.0             # Check that VERSION is 4.0.[2-6], 4.0_HEAD or trunk (all svn), or 4.2.0 (gitlab)
...
COMPONENTS="NEMO OCE"          # Which NEMO components to build OCE/ICE/TOP etc. Enter as list with spaces following (optional) cfg name, E.g. 'CFG_NAME OCE TOP'
CPP_KEYS="key_diainstant key_xios key_qco"     # Any additional compiler keys to include? E.g. from 4.2.x

In practise can not find a way to add multiple entries (e.g. key_diainstant AND key_xios) to a variable :-(

NB these two changes can not really be separated as the cpp_keys change with 4.2.0

[jdha]

don't use 2.5 ... use something like the following I've added for JMMP/CO_AMM7_4.2:
4.2) svn checkout http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/trunk@2379 xios
;;
4.2.0) svn checkout http://forge.ipsl.jussieu.fr/ioserver/svn/XIOS/trunk@2379 xios

[jpolton]

I've fixed the namelist reading issues in this branch/PR.
I've left the NEMO / XIOS version edits alone (though reverting to main would help with the merging), pending @jdha modifications already explored.