Enable conversions to be included in the interface

[dorchard]

Allow an explicit conversion function to be given, e.g., this would enable the following to be abstracted in the interface:

rsn = gipl_compose.get_value("snowpack__mass-per-volume_density", units="g cm-3")
gipl_compose.set_value("snow__thermal_conductivity", rsn * rsn * 2.846)

[dorchard]

[["snowpack__mass-per-volume_density"], "snow__thermal_conductivity", lambda mpvd: (mpvd / 100**3) * (mpvd / 100**3) * 2.846]

list[Tuple[list[str], str, Any]]

where the Any is really like: Callable[[float,...], float but not sure mypy can do anything quite like that (needs depdendent types)`