refactor: remove dead implicit_solver and broken PlotProps

implicit_solver ended with raise NotImplementedError() and could never
produce results. PlotProps imported a nonexistent 'vanGenuchten' module.
Unknown solver names now raise ValueError instead of silently falling
through to rk4.

Author: pycckuu

porousmedialab/lab.py

@@ -328,9 +328,6 @@ def reactions_integrate_scipy(self, i):
             i {int} -- step in time
         """
 
-        # C_new, rates_per_elem, rates_per_rate = desolver.ode_integrate(self.profiles, self.dcdt, self.rates, self.constants, self.dt, solver='rk4')
-        # C_new, rates_per_elem = desolver.ode_integrate(self.profiles, self.dcdt, self.rates, self.constants, self.dt, solver='rk4')
-        # for idx_j in range(self.N):
         yinit = np.zeros(len(self.species))  # Pre-allocate outside loop
         for idx_j in range(self.N):
             for idx, s in enumerate(self.species):