[Task]: Make `Stem dynamics` differentiable and efficient

[SarahAlidoost]

The module Stem dynamics has one class WOFOST_Stem_Dynamics that should be changed. See explanations and requirements in #39.

[SCiarella]

@SarahAlidoost I can not find the test files for this specific module. Do we have them somewhere?

[SarahAlidoost]

@SarahAlidoost I can not find the test files for this specific module. Do we have them somewhere?

You are right! I could not also find the stem module in run_tests of pcse. Let's ask Allard.