Changes needed for the VarBc code sprint at JCSDA

[mikecooke77]

This PR updates the varbc2jedi code which takes the varbc coeffs from var and converts them to jedi code. JCSDA have changed how the data is stored in the file and the names. This PR fixes this and should be co-ordinately with these prs:

• [x] https://github.com/JCSDA-internal/ufo/pull/3015
• [x] https://github.com/MetOffice/varpy/pull/41
• [x] https://github.com/MetOffice/jjdocs/pull/152
• [x] https://github.com/MetOffice/malak/pull/111
• [x] https://github.com/MetOffice/sith/pull/341

I have run sith testing and the results can be seen here:

• [x] mobundle testing
• [x] sith testing