Move NeighborPackagedData into cpp files & allow volume tags in dg boundary terms

[nilsdeppe]

Proposed changes

• Some simplifications and moving code to cpp files. This is most of the line changes.
• Allow getting volume tags in dg_boundary_terms functions. This is necessary for fancier Riemann solvers like GFORCE that require a primitive recovery in the boundary terms computation.

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• The code is documented and the documentation renders correctly. Run
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