[WIP] Iterators for path states

[henrik-wolf]

As indicated in issue #217, I started working on an AbstractPathIterator, to loop over all paths encoded in an AbstractPathState. Currently, only one subtype, FloydWarshallIterator is implemented and tested. I would very much like your feedback on the approach taken here, so that I can start implementing something similar for the other PathStates, if possible.
Here is a short list of the thinks I have done:

• Added AbstractPathIterator as supertype to all path iterators (It felt like a very big decision to make the AbstractPathState or subtypes themselves iterable. I feel that would make sense. What do you think?)
• Added FloydWarshallIterator as a wrapper around the AbstractPathState. Now, if you want to iterate over all paths, you need to somehow awkwardly wrap your state in a FloydWarshallIterator(state).
• Added iterate methods needed for iteration
• Added enumerate_path_into! method, which writes a path into a pre-allocated array and returns a view, dramatically cutting down on allocations, compared to enumerate_paths
• added size, length and collect for this iterator. collect returns a matrix that works in the same way as the matrices in the state, that is, matrix[s,d] is the path from s to d. (this feels more logical, than the current output from enumerate_paths)
• While I was at it, I reworked enumerate_paths a bit to cut down on allocations in general, but also to make a request for a path with given start and destination not do the extra work of calculating all paths from start and then returning only the one to destination

While I am happy with the changes to enumerate_paths, I would very much like your feedback on the decisions I took with the iterator part.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.44%. Comparing base (c4360cf) to head (36bcc17).
⚠️ Report is 116 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #221      +/-   ##
==========================================
+ Coverage   97.43%   97.44%   +0.01%     
==========================================
  Files         113      113              
  Lines        6554     6583      +29     
==========================================
+ Hits         6386     6415      +29     
  Misses        168      168              

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