Update dimensionless units in CABLE offline output to be CF-compliant

[SeanBryan51]

Variables in the CABLE offline model output which are dimensionless have their units attribute set to - which does not comply with the CF-conventions (see units section). The units attribute for dimensionless variables should have units set to 1 or have no units attribute defined.

[SeanBryan51]

This is required for CABLE output to work directly with ILAMB (#210). See CABLE-LSM/benchcab-ilamb-config#1 for more details.

[SeanBryan51]

It looks like CABLE currently is working with the ALMA standard for its input and output files which sets the units attribute for dimensionless quantities to -. I think there needs to be a discussion in the community if the ALMA standard should be superseded by the CF conventions.

[SeanBryan51]

ALMA conventions also documented here: https://modelevaluation.org/variableStandards

[SeanBryan51]

Gab (@gabsun) informed me that the ALMA standard is typically updated on a per MIP basis and suggested that ALMA could be updated to be CF compliant.