Pruning n slack

This implements constraint pruning for n-slack cutting plane.
Also some misc improvements.
This really makes the n-slack variant practicable, in particular for large datasets.
The default values are not all that tested, but seems ok.

This needs some further tests.
Also: I implemented it a bit differently than in the 1-slack variant, I think a bit more directly actually.
( i count how long it was since a constraint was active, instead of storing all previous alphas).