This is a bug fix release of simuPOP with the following new features and bug fixes:
* https://github.com/BoPeng/simuPOP/issues/28: Fix a segmentation fault when providing a non-existent VSP index
* https://github.com/BoPeng/simuPOP/issues/31: Allow operator DiscardIf to accept a probability in addition to `True`/`False`
* https://github.com/BoPeng/simuPOP/issues/35: Add a `weightBy` parameter to allow `HeteroMating` to produce offspring subpopulation with weights determined by not only the size of the parental subpopulation, but also by for example number of mating pairs.
* https://github.com/BoPeng/simuPOP/issues/49: Fix output of loci positions in MS and other formats.