Add diagnostic plots of PM-equation

In notebooks/theory/theory.ipynb, add references to the origins of definitions of expressions and equations, and add diagnostic plots to make sure that the PM-equation behaves as expected. See e.g. notebooks/Finished_project/Ws_sensitivity for strange behaviour.

For example, the following seems odd:

class VH(Variable):
    """Vegetation height"""
    expr = z/(0.66+0.1*exp(kappa*w/u_star))

w (wind speed) and u_star (shear stress velocity), are measured variables, whereas vegetation height is a site-specific constant.