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)
  • Get the Scalar type from somewhere else

Fixes #687 (closed)