Support for DG operators in HOG

Overview of TODOs:

• volume-DoF indexing: implement the indexing functions for volume DoFs in the HFG
• interface loop: generate a loop that iterates the neighbors of the current micro
• neighborhood-info: draw collecting infos about neighboring elements (normals, opposite vertex) into generator, generate local variables for necessary quantities (e.g. normal)
• Big difficulty: detect whether an interface is located on a boundary/between macros/just between micros (this is done by NeighborhoodInfo with HyTeG function calls)
• Arbitrary order Lagrangian space: define basis functions with arbitrary order
• ghostlayer indexing: implement the indexing functions for volume-ghostlayers in the HFG
• Difficulty for vectorization: elements do vastly different computation depending on their position (macro-macro boundary + access of ghostlayer DoFs, Dirichlet boundary)

Possible changes to HyTeG DG:

• different loop pattern: iterate volumes and interfaces separately (Kronbichler says its bad)
• iterate inner interfaces and boundary/macro-macro interfaces separately (then all iterations do the same thing, no more cases)