Don't expose internal data in Facet interfaces

I started working on a sparse implementation of 3D facet data, because in some cases only very litte data in the 3D volume is needed.

This posed the challenge to implement `getInternal()` and `set(array)` which doesn't work too well (without allocating a new array).

I therefore propose to remove the two low-level methods from the root interfaces. This is easy to realize and makes it easier to provide alternative implementations. The methods are still present in the base implementations so that the existing classes remain as they are now.