Cache model results for maps

[dbrakenhoff]

It would be nice to cache the results of pstore.maps.modelstats since it can take a while to compute the statistics for all models.

This cache needs to then also be cleared when models change. Not sure if this is possible but worth investigating.

[martinvonk]

I think exposing _plotmap_dataframe (#165) in combination with the parallel computation of the modelstats already helps a lot.