Fix .blend import and rebase regression database

BlenderBMeshConverter was erroneously asserting that it contained a valid
mesh upon construction. The assertion should only occur when
triangulation is requested, allowing non-BMesh blender meshes to load
properly.

Also rebase the regression test suite, validating that .blend meshes
now properly load and also pulling in a number of unexpected success
cases (including meshes of bvh, nff, ifc and fbx types).

This effectively resolves #382.