Added a few tests to PrismMesh
Added check for non-integer shape in meshes

PrismMesh and TesseroidMesh now raise an exception if passed non-integer
shapes.

Fixes #98
Added more tests to TesseroidMesh while I'm at it

The same tests used for PrismMesh but converted for tesseroids
Merge pull request #104 from leouieda/mesh-shape

PrismMesh and TesseroidMesh raise exception when passed non-integer shapes