Starting 3D Equivalent Staggered Grid.

Added 3D support for source classes.
Changed cookbook examples to follow source modifications.
Created ICoord3 and ICoord, could be added to `fatiando.mesher` package.

Added _add_pad 3D capability
Added scalar3_esg
Added _step_scala3_esg

Missing:
apply_dumping3
scalar_reflexive3