Make working in log conductivity a bit more explicit.

Should be fixed in the actual implementation, and use SimPEG maps
instead of forcing the user to work in weird units.

docs/examples/richards_comparisonToCeiliaEtAl1990.py

@@ -9,6 +9,7 @@ import matplotlib.pyplot as plt
 M = Mesh.TensorMesh([np.ones(40)])
 M.setCellGradBC('dirichlet')
 params = Richards.Empirical.HaverkampParams().celia1990
+params['Ks'] = np.log(params['Ks'])
 E = Richards.Empirical.Haverkamp(M, **params)
 
 bc = np.array([-61.5,-20.7])