Support heightmap mesh generation

[rsrasmu2]

Is your feature request related to a problem? Please describe.
There's currently no easy way to build a hexagonal heightmap mesh using Hexx. My solution right now uses the ColumnMeshBuilder to create individual hexagon columns, but that generates geometry for all six sides regardless of whether the sides are ever actually visible.

Describe the solution you'd like
Add the ability to generate a single heightmap.

Describe alternatives you've considered
Modify ColumnMeshBuilder to allow each side to have an individual height, including a height of 0. This would give users a bit more control over heightmap mesh creation at the cost of a small bit of added complexity.

[ManevilleF]

Hi @rsrasmu2 I have an open PR #170 which brings a new heightmap builder which should fit your use case.

Could you try it out and give some feedback ?