Improve flux variables cache filler handling
- Make the filler alias private in the caches (it's just an implementation policy)
- Make filler a property
- Extend interface of empty cache / filler
- If you want an empty cache (like for staggered-navierstokes) just set the cache and filler to the empty placeholders
- Removes the cache implementation for freeflow (replaced by empty cache)
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Get the Scalar type from somewhere else
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Fixes #687 (closed)