Fwdpy11

This is a major new feature release. Finally, we have support for demographic events involving multiple demes, meaning that merges, splits, size changes, migration, etc. are all possible now.

This release incorporates changes from several PRs. See the change log in the [manual](https://fwdpy11.readthedocs.io) for details.

Future release candidates will focus on improving the docs and fixing any bugs that get found along the way.

Fixed 362 and 363

Note: these changes could result in different results being obtained when mutating tree sequences **after** restoring a population from a file. Previous results were correct in distribution, however. The bug fixes mean that an internal lookup table is now properly restored after converting a population back from binary. This restoration means that the output from `fwdpy11.infinite_sites` could now differ from earlier versions.

Fixes 350

The following two PR are the main ones:

* 328 allows neutral mutations during simulations with tree sequences
* 237 adds infrastructure for general demographic models involving discrete demes

The stuff in 237 is *not* usable in simulations yet. That'll come in the next release...

Several new features + bug fixes. See the [milestone](https://github.com/molpopgen/fwdpy11/milestone/17) for details.

Issues fixed: 268, 269, 267

New features: 272, 275, 279