Fix MLFSIterator, add tests for it

[THinnerichs]

Also:

• fixed enumeration order for DFS
• I added tests for DFS because there were none previously.

MLFSIterator requires Herb-AI/HerbGrammar.jl#104 to work.

[THinnerichs]

Iteration is quite slow for now.

[ReubenJ]

Nice catch on the top-down iterator! Once we update the docstring, I think these changes are looking good 👍

[ReubenJ]

Iteration is quite slow for now.

Is this slower as a result of this PR?

[THinnerichs]

is this slower as a rsult of this PR?

The previous was written before Bart, and thus didn't account for uniform trees, and is thus wrong.
I updated the probability calculation, reducing enumeration time to 14.5 seconds in the case above.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.95%. Comparing base (7af0f7e) to head (f54e32a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
+ Coverage   72.94%   73.95%   +1.01%     
==========================================
  Files          19       19              
  Lines         669      672       +3     
==========================================
+ Hits          488      497       +9     
+ Misses        181      175       -6     

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