Major update with lots of cool new stuff...

Bug fixes:

• Fixed bug in :func:fwdpy11.sampling.DataMatrix.selected that returned wrong data in best case scenario and could
  have caused crash in worst case.
  (e715fb7).
• Fix bug recording fixation times. If a population was evolved multiple times, fixation times from the later rounds of
  evolution were incorrect.
  (9db14d8)
• Fix issue #1, related to fixations in quantitative trait sims. (6a27386)
• The "label" field of a diploid is now initialized upon constructing a population.

API and back-end changes:

• Added :func:fwdpy11.sampling.matrix_to_sample and :func:fwdpy11.sampling.separate_samples_by_loci. (i639c8de999679140fad6a976ff6c1996b25444aa)
• Custom stateless fitness/genetic value calculations may now be implemented with a minimal amount of C++ code. See
  :ref:customgvaluecpp.
  (a75166d)
• Custom fitness/genetic value calculations now allowed in pure Python, but they are quite slow (for now). See
  :ref:customgvalues. (5549286)
• Stateful trait value models enabled for qtrait sims. (161dfce)
• Refactor evolution functions so that stateful fitness models behave as expected. Enable compiling in a debug mode.
  Fix bug in operator== for diploid type. (a726c05)
• fwdpy11.util added, providing :func:fwdpy11.util.add_mutation. (17b92db)
• Simulations now parameterized using classes in fwdpy11.model_params. (18e261c) and (eda7390)
• Added multi-locus simulation of quantitative traits.(fcad8de)
• Refactoring of type names. (632477c)
• Refactoring internals of single-region fitnes/trait value typess. (d55d636)
• Allow selected mutations to be retained in fwdpy11.wright_fisher.evolve_regions_sampler_fitness. (dcc1f2f)