Is your feature request related to a problem?

Currently it looks like Index.create_variables cannot create a new variable, i.e. it must return a subset of variables passed in set_xindex. Any extra variables returned are ignored.

I am working with a ForecastIndex for weather forecast data cubes that have two dimensions time, step. I'd like the index to return a valid_time = time + step and for ForecastIndex to be associated with time, step, and valid_time.

1. Is there an argument for loosening this restriction?
2. If we don't loosen it, then I think we should raise an error, instead of silently ignoring any extra variables (very confusing).