Allow to split nodes differently

[DerWeh]

It would be good to give control over the way split_node splits the node. Oftentimes, we want to push the singular values left or right, to move the center of orthogonality while we split the node (e.g. in the standard 2-site DMRG).

To my knowledge, currently split_node_full_svd has to be used in this case, which is kind of roundabout.