Adds NoTears structure learning algorithm

[ankurankan]

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[codecov]

Codecov Report

Merging #1354 (055ffea) into dev (836a44c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1354   +/-   ##
=======================================
  Coverage   93.19%   93.19%           
=======================================
  Files         132      132           
  Lines       13250    13250           
=======================================
  Hits        12348    12348           
  Misses        902      902           

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