Fwdpy11

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

This is an intermediate release as we are still working towards supporting more general demographic models.

Major changes include:

* Updating the fwdpp back-end to the pre-release code for fwdpp 0.8.0. Almost none of these changes are "user facing".
* Add :class:`fwdpy11.SiteTable`, :class:`fwdpy11.Site` and new fields to :class:`fwdpy11.MutationRecord`. 258 These changes affect the API for some function calls. See :ref:`upgrade_path` for details.

Even though this release changes some of the tree sequence data structures, we are still able to read in files generated by version 0.4.5! (This is actually unit tested.)

Minor changes include:

* Add :func:`fwdpy11.gsl_version`. :class:`fwdpy11.MutationRecord`.256
* :attr:`fwdpy11.Mutation.g` is converted to the mutation's age when dumping table collections to tskit's format. 257
* New exception types from fwdpp registered as Python exceptions. 260
* Several updates to documentation and to continuous integration testing.

Fixes handling of nested window coordinates. 244

Added `fwdpy11.DataMatrixIterator`, which allows very fast traversal of multiple genomic intervals, retrieving the genotype matrix at each window.