First pass implementation of holes :

- make sure HBR passes down the hole tag to children when subdividing faces
- minor API modification : allow to unset the hole flag on a face
- modify uniform / adaptive FarMeshFactory to be aware of the flag
- make the FarSubdivisionTableFactory assert when finding unconnected HBR vertices (as it should)

* Uniform subdivision :
    The refinement scheme only creates faces & vertices necessary
    to maintain the one-ring around the edges of a hole, so this solution
    is quite efficient.

* Adaptive subdivision :
    At the moment we are still performing full topological analysis on holes and
    only skipping patches associated to holes. This is sub-optimal in 2 ways :
        1. the topological analysis can potentially be cranking on a lot of unnecessary
           geometry
        2. even though we may not be drawing the patches, the compute stage is still
           applying kernels on all the control vertices of these patches.
    We will have to revisit feature adaptive subdivision & holes, so keep the issue active.

fixes #78