Generalized Morton SFC based Adaptivity (Generalized Element Type)

[koehlerson]

Right now our p4est implementation is based on quad/hex morton index. It should be possible to come up with a generalized morton index contract to incorporate also triangles, tets, wedges whatever:

• https://arxiv.org/abs/1509.04627
• https://bonndoc.ulb.uni-bonn.de/xmlui/handle/20.500.11811/7661