Begin migration to NEMO v4.2

• Update setup script to pull from NEMO gitlab (@jdha - done in other repos)
• Re-organise repo structure to accommodate different versions of NEMO (@jdha - done in other repos)
• Migrate 4.0.4 namelist_cfg to 4.2 (@anwiseNOCL ?)

Testing

• switch off LBCs, SBCs, ICs run with CO9 domain_cfg and usr_def_istate
• switch on LBCs (equal to ICs)
  ✔️ 1. with 'old' bdy coordinates files used in previous version
  2. with 'new' bdy coordinates files created by PyNemo
• add in JRA SBCs and document setup (scripts to assist?)
• add in realistic (GO8p6) ICs and document setup (scripts to assist?)
• add in realistic (GO8p6) LBCs and document setup (scripts to assist?)
• switch on tides
• add option to setup script (and test) for XIOS3 for NEMO>=4.2.0

Things to bare in mind

We start with an empty MY_SRC. This may have to change to get things running. Once we have a working version a discussion needs be had about what, if anything, additions need porting to 4.2.