Allow flux reconstruction for nonconforming grids
The following discussion from !105 (merged) should be addressed:

@sospinar started a discussion: (+6 comments)
I have been thinking about the nonconforming grids for flux reconstruction. It is possible, but it is absolutely needed to do something more than local flux reconstruction (that is what this MR is about):
One has to solve the same problem but globally, and constraint the normal fluxes to be equal on the intersections. This procedure leads to the socalled hanging nodes. This is not needed for the conforming grids because normal fluxes are trivially equal for RT elements of the same order. In such a case, each grid entity has all the information to do the flux reconstruction, and computations become local. (Notice that padaptivity for the flux reconstruction also needs to solve the global problem).
In that case, some of the things implemented here need to be modified:
 Form a global linear system rather than a local linear system. It means that the Raviart Thomas local engine has to be modified so that it forms the right linear system. (Or just creating another local engine).
 Local Function Spaces (LFS) cannot be the one I implemented here: It can be either that the LFS of PDELab receives intersections, or that the
MinimalLocalFunctionSpace
can construct the DOF and can form a composite space (yeah, it implies usingTypeTree
).  One has to constrain the degrees of freedom associated to the entities of codimension 1 in a very similar way as for hanging nodes in normal FEM (PDELab has a quite old code for it, not sure if it would work).
 As well as for this MR, the lifting can be done separately (e.g. with this MR infrastructure), or directly together with the flux reconstruction RT elements.
 Check again the computations in the local operator for the nonconforming Raviart Thomas finite elements (and its test functions).
Since it is quite a lot of work I won't do what I just described and since local flux reconstruction in nonconforming grids is not useful at all for us, and I will disable local flux reconstruction computation for the cubeadaptative cases. @lriedel Are you fine with it?