Description of the desired feature

Implement grdmix which does "Blending and transforming grids and images".

Inspired by GenericMappingTools/gmt#3992 (comment) which would help resolve the longstanding PR at #370 (comment)

Implementation wise, should we output the rgb raster into an xarray.Dataset? Or a list of xarray.DataArrays? See also related discussion at #370.

Are you willing to help implement and maintain this feature? Yes, but help is welcome