'Frame' is additional faces for HexMesh going downward, and Prism has
additional faces which goes upward. Factor out commond code, add trivial
classes: Triangle, Edge, Face
Fix calculation of PrismHexMesh's height
Previously it was calculated correctly only for polyhedron
Unify construction of faces for HexMesh and Prism
'Frame' is additional faces for HexMesh going downward, and Prism has additional faces which goes upward. Factor out commond code, add trivial classes: Triangle, Edge, Face
Fix calculation of PrismHexMesh's height
Previously it was calculated correctly only for polyhedron
Add function for PrismHexMesh construction
Missed previously
Resolves: #18