Unify implementations of sweeping algorithms

[milan-hl]

Sweeping algorithms:

1. SVD compression
2. Variational compression
3. Minimal eigenvalue computation
   Number 2 and 3 may involve operations on supersites which need to be split after being modified (in the same way in both cases).

Possible goal for unified implementation: Make adaptive versions of the new implementation easy to implement. (E.g. using the compression error when splitting supersites, or other values.)