Add high-level python API for tree-sequence metadata

[benjeffery]

Resolves #534
As we are doing a metadata docs pass in #603 I have only done minimal docs.

[codecov]

Codecov Report

Merging #666 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #666      +/-   ##
==========================================
+ Coverage   87.74%   87.75%   +0.01%     
==========================================
  Files          23       23              
  Lines       17668    17687      +19     
  Branches     3498     3498              
==========================================
+ Hits        15503    15522      +19     
  Misses       1062     1062              
  Partials     1103     1103              

Flag	Coverage Δ
#c_tests	89.03% <100.00%> (+0.01%)	⬆️
#python_c_tests	91.23% <100.00%> (+0.01%)	⬆️
#python_tests	98.99% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
python/tskit/tables.py	99.66% <100.00%> (+<0.01%)	⬆️
python/tskit/trees.py	98.21% <100.00%> (+<0.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5958f0e...f96a4db. Read the comment docs.

[jeromekelleher]

LGTM, minor comments.

[jeromekelleher]

I don't like this as it makes the tests stateful and the test data will depend on what other tests are run on this class. Also adding instance variables to a TestCase is a bit weird. Either put in some constants, or make val a parameter.

[benjeffery]

I made it a param.

[benjeffery]

@jeromekelleher Fixed up - given the number of this PR I got off lightly. 👿

[jeromekelleher]

Not an Iron Maiden fan then? I keep singing "6,66 the PR of the Beast" to myself. Classic.

[benjeffery]

Travis AWOL again - will look into this as happening a lot.