File input for mixed topology

[chrisrichardson]

Example of reading mixed topology:

https://gist.github.com/jorgensd/fcbcee61f773e8889eccacf2f75158a2
(needs mesh_converter installed: https://github.com/jorgensd/mesh_converter/
test.vtkhdf.zip

Issue
I can't get this branch to work with mixing P1 and P2 triangles in the same mesh (even if they are completely disconnected.

If i change the order of the second triangle to 1, the code runs smoothly.
I do not have minimal working example. But I've attached the mesh I've used here, and the code I've used for reading the mesh is available at:

[mscroggs]

see also #3231 and #3236

[jorgensd]

See also: #3271 (comment) for a prototype of this