Replacing matplotlib.mlab.griddata by scipy.interpolate.griddata

As discussed in issue #146, I'd some problems when I tried to run `PrismMesh.carvetopo` because of `matplotlib.mlab.griddata`. So, @leouieda suggested to replace the `matplotlib.mlab.griddata` functions  by `scipy.interpolate.griddata`.

@leouieda I don't know what's the best interpolation method to set as default. With `scipy.interpolate.griddata` our options are `linear`, `nearest` and `cubic`. Here I'm using `cubic`.