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
• Come up with representation description
  • Make structure mutable
  • Allow reusing memory
  • Make the graph irredundant (each node represents a single free band element)
• Add methods to initialize FreeBandElementByGraph
  • Initialize from word
  • Initialize from tree-rep
  • Initialize from graph
• Multiplication
  • Multiply by single letter
  • Figure out how to multiply two elements in graph-rep
• Equality checking
• Canonical word
  • Exponential canon. word
  • A shorter word-rep due to @tomcontileslie
• Making a copy (to support immutable operations)
• 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