WIP: allow periodic triangular grids

Implements the refinebalance and split_bisection method for periodic grids. Testing in test-alugrid is included.

Open Problems:

• Correct IdentifyFaces behavior in 2d with tets
• Before Coarsening, set edgelock flags correctly across periodic elements

Problem description:

• The 2d triangle part of the identifyFaces method does not seem to have ever been tested/used. It apparently does not identify all periodic faces correctly
• The coarse() call in gitter_pll_sti makes the grid non-conforming across periodic elements, so the edge locks are not set correctly