migration to being functional

[petrelharp]

Here's the things that need to be dealt with to get rid of SlimTreeSequence:

Attributes/methods of the SlimTreeSequence to be moved to pyslim.X functions:

• recapitate here
• mutation_at
• nucleotide_at
• individuals_alive_at
• individual_ages_at
• slim_time
• individual_parents/has_individual_parents
• individual_locations/individual_ages/individual_times/individual_populations

Things to be deprecated/moved to metadata:

• reference_sequence
• slim_generation
• model_type
• slim_provenance

Those that are wrappers around tskit and so we can omit entirely are:

• dump/load

[petrelharp]

See #209 for some of these.

[petrelharp]

And more in #231 and #230.

[petrelharp]

The remainder of this will happen in v1.0.

[petrelharp]

This is done!