CMIP7 cell methods and variable mapping review

[rbeucher]

Cell methods and variable mappings (input_vars, calculation) have changed for CMIP7.

Requesting review of recent updates against the CMIP7 spreadsheet:
https://docs.google.com/spreadsheets/d/1VTKrG9XcwPOVD7i-XWE2vbphm_QvSk61/edit?gid=569180724#gid=569180724&range=A44:N76