Add area-weighted resampling

I was looking at the map projection resampling documentation and none of the methods are flux conserving. As there is a flux conserving (area weighted) resampling in the L3 processor, it might be a quick win to add this as a new method.

The interpolation methods cause a blurring (e.g. at the coastline, see http://www.brockmann-consult.de/beam/doc/help/general/ResamplingMethods.html).

This is unavoidable where there are continuous values but where a bitmask is applied, an area weighting can be used to avoid this averaging affect. This technique is used in the L3 processor flux conserving resampling.