Reduce StaticCache compile time

[nilsdeppe]

Proposed changes

Some timings on mbot:

Timings for DgSubcell/Matrices.cpp
 Original: 46
 Combine static cache generate_tuple functions: 46
 Move integral static cache range check: 40
 Change StaticCache enum indexing and bounds check:  22
 Combine enum and integer caching logic in StaticCache: 20
Improve StaticCache compilation time using multiple peelings: 10
Simplifying DgSubcell matrix caching code: 12

The last commit simplifies the code quite a bit, IMO, which is why I did it.

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