Free band elements by graph representation

[reiniscirpons]

This PR adds some functions for creating and manipulating free band elements in a graph representation. This representation has benefits in comparison to the current tree based rep, and even a possible word based one.

• [ ] Actually describe what the structure and algorithms are
• [x] Come up with representation description
  • [x] Make structure mutable
  • [x] Allow reusing memory
  • [x] Make the graph irredundant (each node represents a single free band element)
• [ ] Add methods to initialize FreeBandElementByGraph
  • [x] Initialize from word
  • [ ] Initialize from tree-rep
  • [ ] Initialize from graph
• [ ] Multiplication
  • [x] Multiply by single letter
  • [ ] Figure out how to multiply two elements in graph-rep
• [x] Equality checking
• [x] Canonical word
  • [x] Exponential canon. word
  • [x] A shorter word-rep due to @tomcontileslie
• [x] Making a copy (to support immutable operations)
• [x] DotString for visualizing the structure
• [ ] Iterators (?)
• [ ] Collections of elements
  • [ ] Reuse element code to make collection, redefine elements be specified by a collection and node in collection
• [ ] Write up a proof for certain results
  • [ ] V <= |A|*N where V is the number of vertices of the graph, N is the length of the shortest word representation and |A| is the size of the alphabet
  • [ ] The right multiplication by letter algorithm is correct
  • [ ] The canonical word algorithm actually produces the shortest possible word