Major feature release.

This is a major feature release. The changes include all those listed for the various
release candidates (0.6.0rc0, 0.6.0rc1, and 0.6.0rc2) plus the following:

• Several back-end issues are fixed:
  Issue 388
  Issue 389
  Issue 390
  Issue 392
• fwdpy11.TableCollection.fs added.
  PR387
  PR399
• Creating populations from msprime input improved.
  PR395
• Added PendingDeprecationWarning to fwdpy11.evolve_genomes.
  PR396