Adding and removing multiple solutions in an ensemble takes too long

[sforsten]

If you add or remove multiple solutions in an ensemble, it takes a lot of time, because after adding/removing one solution the ensemble recalculates. Although the ensemble would have methods to add/remove multiple solutions at once, the view (ClassificationEnsembleSolutionModelView, RegressionEnsembleSolutionModelView)only removes one solution at a time.

[sforsten]

r8719: if possible all solutions are added/removed at once, otherwise one solution is added/removed at a time