Warning to update preprocessed grids

[mdtanker]

From #256, in the most recent version, new preprocessing for some grids (ie. Bedmap2), has used new variable names. If users retain their old preprocessed grids from past versions, the fetch functions may expect incorrect variable names, raising an attribute error. We should warn users to clear their pooch caches when they update versions, and we should catch attribute errors and give users a warning about this.