Add metadata schema methods to Python C module

[benjeffery]

As #491 was getting very large we decided to split it again. This PR adds methods to the Python C module to get/set metadata schema as a *Table attribute, and to the get the group of schemas as a method on TreeSequence

[codecov]

Codecov Report

Merging #542 into master will decrease coverage by 0.10%.
The diff coverage is 76.82%.

@@            Coverage Diff             @@
##           master     #542      +/-   ##
==========================================
- Coverage   87.26%   87.16%   -0.11%     
==========================================
  Files          21       21              
  Lines       16037    16200     +163     
  Branches     3132     3172      +40     
==========================================
+ Hits        13995    14120     +125     
- Misses       1006     1020      +14     
- Partials     1036     1060      +24     

Flag	Coverage Δ
#c_tests	88.33% <ø> (ø)
#python_c_tests	90.24% <76.82%> (-0.31%)	⬇️
#python_tests	?

Impacted Files	Coverage Δ
python/_tskitmodule.c	83.93% <76.82%> (-0.29%)	⬇️
python/tskit/trees.py	98.60% <0.00%> (ø)

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[jeromekelleher]

Looks great, thanks @benjeffery. A couple of minor comments above, and I think we're good to merge.

Can you confirm that python3 stress_lowlevel.py -m lowlevel shows we have no leaks?

[benjeffery]

@jeromekelleher looks good to me:

(env) benj@treebeard:~/projects/tskit/python$ python3 stress_lowlevel.py -m lowlevel
iter    tests    err    fail    skip	RSS      min    max      max@iter
317     166      0	    0       0	    78668    78336	78668    2

If you're happy I'll squash rebase.

[jeromekelleher]

Great thanks, let's merge.

[benjeffery]

Travis failure is on push trigger only. pr trigger is passing.