pagmo 2.0

This is the initial release of pagmo version 2.

pagmo 2 is a new, improved rewrite of pagmo 1.x, featuring better stability, a superior python experience and first class Windows support. See https://github.com/esa/pagmo2/wiki/From-1.x-to-2.x

NOTE: not all features from pagmo 1.x have been ported over to pagmo 2 yet. We have planned a rapid release schedule through which we will be adding back the pagmo 1.x features currently missing in pagmo 2.