Force top-level of metadata to be an object

[benjeffery]

Fixes #634

[codecov]

Codecov Report

Merging #639 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #639   +/-   ##
=======================================
  Coverage   87.42%   87.42%           
=======================================
  Files          22       22           
  Lines       16911    16912    +1     
  Branches     3316     3316           
=======================================
+ Hits        14785    14786    +1     
  Misses       1040     1040           
  Partials     1086     1086           

Flag	Coverage Δ
#c_tests	88.64% <100.00%> (+<0.01%)	⬆️
#python_c_tests	90.66% <100.00%> (+<0.01%)	⬆️
#python_tests	98.96% <100.00%> (+<0.01%)	⬆️

Impacted Files	Coverage Δ
python/tskit/metadata.py	98.33% <100.00%> (+<0.01%)	⬆️

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[jeromekelleher]

Looks good. I think we should remove the top-level "SLiM" key in the metadata. Examples are sticky and I don't think this is what we'd encourage as best-practise.