Add helper functions to get stressmodels from pastastore

[dbrakenhoff]

Make it even easier to build Pastas models! As suggested by @FransSchaars.

Build stressmodels from stored stress time series with pstore.get_stressmodels()

sm = pstore.get_stressmodel("well1", rfunc=ps.Hantush, up=False)

Methods support nearest option when x and y-coordinates are stored for all stresses and oseries! Add them directly to pastas models with pstore.add_stressmodel()

ml = ps.Model(oseries)  # or use `pstore.create_model(oseries_name)`
pstore.add_stressmodel(ml, stresses=["nearest prec", "nearest evap"], stressmodel=ps.RechargeModel)

+ some other clean up and improvements

[dbrakenhoff]

Test fail is because i annoyed the KNMI api, so all good to go after review.

[martinvonk]

Maybe we can separate the deletion of the arctic and pystore to another pr such that @FransSchaars can review and test this?

[dbrakenhoff]

Maybe we can separate the deletion of the arctic and pystore to another pr such that @FransSchaars can review and test this?

Just ignore the deletions? It should work fine. (I'm a bit lazy and don't feel like separating the two :).)