Create conversion function for aed or glm variables

[jordansread]

Related to #182, but this is more basic: Provide a way for users to do simple conversions of the output of the model.

One example is output temperature in °F, or DO in mg/L.

Using @jiwalker-usgs 's suggestion to just add them to the netcdf file, this would make them usable by all of the tools that generically work on sim_vars, such as plot_var_compare, plot_var, etc.