New metadata schema, new distribution of effect sizes.

Interface changes:

• When dumping tables to tskit via :func:fwdpy11.DiploidPopulation.dump_tables_to_tskit, we now use the latest metadata methods.
  #588
• Most arguments to :func:fwdpy11.evolvets are now keyword-only.
  #584
• Added :class:fwdpy11.DiscreteDESD.
  #587

Dependency updates:

• Minimum tskit version is now 0.3.2.
• Minimum attrs version is now 0.19.2.
• Sphinx version pinned to 3.1.
• The warnings for deprecated features got promoted to :class:FutureWarning.
  #583

C++ back-end changes:

• New method to handle GSL errors. #574
• Table collections are now managed by shared pointers. #582