Convert to SU(2)

Created by: tobiaslcmueller

SU(N) implementation converted to SU(2).

For spin S we have to think a bit more about the units: currently, the bare vertex is rescaled by 2S, as in the SU(N) formulation. This means, the correlations are in units of |J|/(2S), not |J|, as one would naively expect. Should we change that back within the correlation calculation? Or just document it, as the current behavior is consistent with the unit of the vertices?

Closes #37