Slim v5 updates

[petrelharp]

TODO:

• update metadata schema (again)
• write function to return the proper node metadata schema
• update test recipes
• write tests for num_chromosomes in metadata functions
• add tests of writing and reloading trees archives
• add a method to merge a collection of trees files into a trees archive
• add tests of this, annotating a collection of msprime tree sequences to a trees archive and loading it
• add tests of this, making a collection of SLiM tree sequences a trees archive and loading it
• test converting old versions to the current version
• provide method to remove vacant sample nodes

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.55072% with 2 lines in your changes missing coverage. Please review.

Project coverage is 96.72%. Comparing base (c4082b3) to head (b6db31d).

Files with missing lines	Patch %	Lines
pyslim/slim_metadata.py	96.77%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #367      +/-   ##
==========================================
+ Coverage   95.65%   96.72%   +1.07%     
==========================================
  Files           8        8              
  Lines         621      764     +143     
  Branches      119      145      +26     
==========================================
+ Hits          594      739     +145     
+ Misses         18       15       -3     
- Partials        9       10       +1     

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