Restructured the reading of the high order grids for the FEM solver

[vdweide]

Proposed Changes

Adapted the reading of the higher grids, such that it now uses the various meshreader classes. There is some additional functionality, namely that it is now possible to use rectangular and box grids with the higher order FEM solver, but it is mainly meant to make the reading more consistent with the code infrastructure. For this reason additional base classes have been introduced for the reading of CGNS and SU2ASCII files.

Related Work

This is the first of a couple of PRs to clean up the code for the DG-FEM solver. It does not add any functionality nor it fixes any bugs.

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