Remove StressModel2

[raoulcollenteur]

Short Description

Remove StressModel2 as decided per issue #363.

Checklist before PR can be merged:

• closes issue Deprecate StressModel2 in v0.21.0, remove in v0.23.0 #363
• tests added / passed
• API changes documented in Release Notes

[raoulcollenteur]

Should we throw an error message in pastas/io/base.py when loading a file which still has StressModel2? And then for Pastas v.0.22.0 make sure that people can load and save the .pas-file to the new format.

if ts["stressmodel"] == "StressModel2": 
    warning("StressModel2 is removed since Pastas 0.22.0. Make sure to save this file 
                     using v0.22.0 as this file will not work in ")
    ts["stressmodel"] = "RechargeModel"
    ts["recharge"] = "Linear"
    ts["prec"] = ts["stress"][0]
    ts["evap"] = ts["stress"][1]

[raoulcollenteur]

The above method actually works well, I tested using the following code using notebook 8_threshold_nonlinear.ipynb.

ml = pt.io.load('data_notebook_8/B28H1804_2.pas')
ml.to_file("test.pas")
ml1 = pt.io.load('test.pas')
pt.plots.compare([ml, ml1])

B28H1808_2.pas.zip old pas-file for future reference. I updated the old pas-file to the newer version now.

PR should be good to go now I think. Can you have a look @rubencalje ?