This is the only way to (currently) regularise a reduced grid in the grib loader. The way I've implemented it is that the regularisation interpolation will only occur once in the proxy; so deferred regularisation. However, `_regularise` must be called beforehand to calculate the deferred shape of the data for the proxy (kinda :chicken:  and :egg: )

Sub-optimal, I know, and we could circumvent it. I'd recommend not doing it in this PR. My intention is for this PR to be a big step in the right direction for the GRIB loader with a minimal change. We **really** need to re-factor the grib loader, and the new design should accommodate how reduced grids need to be processed acknowledging the limitations/restrictions of the gribapi and its inner workings.