molpopgen/fwdpp: Tree sequence support

This is a major release, introducing namespace fwdpp::ts with support for tree sequence recording during simulations. I owe Jerome Kelleher a big thanks for guiding me through some of the details related to several of the finer points of the simplification algorithm as well as the tree traversal schemes from his 2016 paper describing msprime.

• Support for tree sequences added PR 151 PR 152
• fwdpp::data_matrix is moved into the main library. PR 118
• fwdpp::data_matrix is refactored so that the contiguous dimension corresponds to variable sites.PR 142
• Fitness models do a better job of validating input parameters.PR 140
• Types in fwdpp::extensions now make their weight vectors viewable. PR 141
• Various pull with some example programs were fixed PR 149
• A debug namespace was added to the library. PR 150