Allow variable units to vary with context

[mdpiper]

Currently, get_var_units returns a static unit for a variable, like "m" or "kg". However, some models have variables with units that vary with context; see, e.g., discussion here.

This is a request to explore how to include context-varying units.

[mdpiper]

Talking with @mcflugen, this is really an implementation detail. We can handle this with the existing get_var_units function.