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