Document how to disable cmor checks and fixes

[bouweandela]

For scientists who like living on the edge

[bouweandela]

To disable the CMOR checks, use the following preprocessor:

fix_metadata: {cmor_table: null}
fix_data: {cmor_table: null}
cmor_check_metadata: false
cmor_check_data: false

the cmor_table argument to fix_(meta)data is needed because part of the automatic fixes is done by the cmor checkers (don't ask me why).

To disable both fixes and CMOR checks:

fix_file: false
fix_metadata: false
fix_data: false
cmor_check_metadata: false
cmor_check_data: false

[valeriupredoi]

superseded and implemented via cmor check levels by #374 - @bouweandela we can close this