Changing the default algorithm for model creation in segment editor

[lassoan (Andras Lasso)]

Still flying edges is used in the Slicer Preview Release. Surface nets is the new, faster option. We agree that we should use that by default (Bad performance when segmenting 11GB micro-CT - #8 by pieper). We haven’t decided if we should do it in Slicer 5.8 or later, but I think it is better to do it now. I’ve added an issue to track this:

Make surface nets the default closed surface generation algorithm