Checking that Interpolator returns a float (and not xarray DataArray)

[erikvansebille]

I just spent an hour chasing an issue in xgrid.search() that turned out to be the result of the custom Interpolator that I used returned a DataArray. This is an easy mistake to make as a user when they don't return the Field.data.value

We should thus check that the return type of any Interpolator is a float

[erikvansebille]

@VeckoTheGecko, is this implemented now that(since #2646) Interpolators are functions that are type-checked?

[VeckoTheGecko]

We don't do runtime typechecking - so its still possible that people write their interpolator wrong