Properly export constants

[hyanwong]

At the moment tskit.NULL comes because we do from _tskit import NULL within trees.py, then in __init.py__ we do from tskit.trees import *. This indirect import seems nasty to me. So I've changed this so that all constants are explicitly exported via a line in __init.py__ instead.

[codecov]

Codecov Report

Merging #267 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #267      +/-   ##
=========================================
+ Coverage    85.8%   85.8%   +<.01%     
=========================================
  Files          19      19              
  Lines       14068   14071       +3     
  Branches     2736    2736              
=========================================
+ Hits        12071   12074       +3     
  Misses       1043    1043              
  Partials      954     954

Flag	Coverage Δ
#c_tests	86.73% <100%> (ø)	⬆️
#python_c_tests	89.94% <100%> (ø)	⬆️
#python_tests	99.17% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
python/tskit/trees.py	98.58% <100%> (ø)	⬆️
python/tskit/__init__.py	100% <100%> (ø)	⬆️

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[jeromekelleher]

LGTM, thanks. will merge after #268.