Implement new vertical coords in AMM15

[jpolton]

Following recommendation from AMM7_zgr work, implement new vertical coordinates in AMM15v4.

Test config:

• short HPG validation/sanity check (unforced and starting from rest)

• Realistic run for 1-2 years (to be discussed)

• Using existing code, assess the skill of the configuration

[jpolton]

See https://github.com/JMMP-Group/AMM7_zgr/wiki for progress in AMM7