[discretization] Add interfaces element/isBound to grid geometry local views

Merged Timo Koch requested to merge feature/localgeometry-element-interface into master

Another option would to use std::optional (storing value vs. storing pointer). Seems like that's what's used for the DefaultLocalView in dune-functions. I guess optional involves a copy for grids like YaspGrid. Can it happen, e.g. in a coupling context object that the element pointed to goes out-of-scope? I guess it can? Then we should definitely use std::optional.

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