Increase npad to reduce numerical error

Increase npad for DC_Pseudosections to reduce numerical error in the
half-space example, so the apparent resistivities are somewhat constant.

geoscilabs/dcip/DC_Pseudosections.py

@@ -58,7 +58,7 @@ def _transform(self, m):
 
 
 # Mesh, mapping can be globals
-npad = 15
+npad = 40
 cs = 1.25
 hx = [(cs, npad, -1.3), (cs, 100), (cs, npad, 1.3)]
 hy = [(cs, npad, -1.3), (cs, 50)]