simpeg/simpeg: enhanced plot2DData and IO.pseudosections

• commits from @thast
• review from @lheagy

Address issue #734

PR notes: We have fixed the following bug:

• the clim argument was not used in the SimPEG.Utils.plot2DData. We added the functionality to modify the colorscale based on the clim argument.

Improvements of the SimPEG.Utils.plot2DData function:

• Levels of the matplotlib.pyplot.contourf function are defined by the function itself. This is done to give us complete control over the colorscale and colorbar.
• Before plotting, we check if any data are NaN or infinite to ensure reasonable colorscale. We also ensure positivity is scale==log
• colorbar limits choice through clim and ncontours: ncontours defines the number of contour inside the clim scale. If data lies outside that range on either side, an supplemental contour is added to regroup those data under a single color on either side (so two colors if on both side with of ncontour+2 contours).