added a new setTopologyUnchecked method to MeshPrimitive, plus lazy computation of min/maxVerticesPerFace
removed 'virtual' from NURBSPrimitive::setTopology while I'm at it
IECoreGL renderer now uses new MeshPrimitive::setTopologyUnchecked() method to save a bit of work
moved some checks into computeMinMaxVertsPerFace(), which now gets called by setTopology()
Merge pull request #106 from davidsminor/meshPrimitiveUncheckedTopology

Mesh primitive unchecked topology