Solve the puzzle/game "Flow Colors" (known under other names as: "Free Flow", "Flow Free").
(These names may be trademarked by their various owners).

The library solves by doing depth first search with back tracking.
Each trail is solved one after the other based on the order that the end points are
specified. Giving the trails in a good order may improve the solving speed.

Various techniques are used for choosing a good search order, and pruning out 
branches that won't lead to a solution.

Performance is satisfactory.

Currently the problem must be specified in coding. The command line could be 
easily extended to support reading the problem specification.

The output is a SVG file showing the solution.

Function GFC_Diagrams.genOutputF searches for a solution without perfect cover
(however all trails must be complete). It is faster than genOutput which backtracks
until it finds  a solution with either perfect cover, or with one square not
covered.