VariableWriter does not import functions used in expr

[schymans]

If I define an expression in a variable definition, VariableWriter does not import it automatically and it has to be added manually as supplementary_imports. In https://github.com/environmentalscience/essm/blob/master/docs/examples/api_features.ipynb, In[14], the following definition was made:

class Delta_Pwa(Variable):
    """Slope of saturated vapour pressure, $\partial P_{wa} / \partial T_g$"""
    expr = Derivative(P_wa,T_g)
    latex_name = r'\Delta'

However, Derivative is not imported when executing:

writer_var = VariableWriter(docstring='Test of VariableWriter.')
writer_var.var(Delta_Pwa)

This is because extract_functions() in https://github.com/environmentalscience/essm/blob/master/essm/_generator.py is only used in EquationWriter but not in VariableWriter.