[Feature]: Update more land variables to support ilamb analysis.

[chengzhuzhang]

Is your feature request related to a problem?

We are short of variable handlers for land variables.

Describe the solution you'd like

Start with an initial list of ilamb needed variables (variables collected by @bon), and later to add more variable to support v3 CMIP submission.:

fBNF - nitrogen fixation [kg m-2 s-1]
snw - surface snow [kg m-2]
tasmin - minimum daily temperature (per month) [K]
tasmax - maximum daily temperature (per month) [K]
hurs - surface relative humidity [%]

Describe alternatives you've considered

No response

Additional context

No response