Add clone and merge candidates

[shreyas-kowshik]

The older merge-candidates code that we were using was sort of a work-around.
This code is more updated. But it may have some bugs and we are currently figuring them out in our work as well.

[codecov-commenter]

Codecov Report

Merging #31 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage   95.00%   95.06%   +0.06%     
==========================================
  Files          23       23              
  Lines        1640     1660      +20     
==========================================
+ Hits         1558     1578      +20     
  Misses         82       82              

Impacted Files	Coverage Δ
src/transformations.jl	98.47% <100.00%> (+0.17%)	⬆️

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[guyvdbroeck]

Thanks, this is certainly a useful addition. Any chance we can have a small unit test for it?

[shreyas-kowshik]

Let me add tests for clone_candidates.
For merge_candidates I believe the probabilistic-checking needs fixes as you pointed out and so perhaps we should add them after that is done?

[guyvdbroeck]

Let's indeed leave out merge_candidates for now, it has other concerns, for example that it requires a vtree.
Also then our code coverage does not decrease.
The clone_candidates looks good as is. In future, we'd like to avoid having to use Dict for this, since it's slow. One thing you could do is use the ability to count parents in graphs.jl by doing a foreach without resetting the node.counter field. Then you can do a filter based on the number of parents. Not sure if it would be faster, so not a priority.
If you remove the merge_candidates I will accept the pull request.

[shreyas-kowshik]

Removed merge_candidates