Add backtracking feature

[eyal0]

--backtrack does a greedy search among line segments looking for
vertices that have:

• odd number of edges
• more outbound edges than inbound and bidirectional
• more inbound edges than outbound and bidirectional

For each one, it finds the nearest other vertex for which duplicating
edges would create a euler circuit. Edges are only added if milling
them is faster than retracting, moving, and lowering.

[eyal0]

Before:

After:

Notice how the length milled has become longer but there are no more retracts. Each one was calculated so that it was only added if milling was faster than retract+g0+lower.

[eyal0]

Here are the paths that are doubled:

It's hard for me to tell if those were good choices. I just have to trust that the algorithm did it right. Definitely we could have made different choices if we didn't use a greedy algorithm but this is a good start. Maybe in the future I can use an algorithm that searches the space of choices, like a minimum weight matching. Code for that is the Blossom V algorithm.

[coveralls]

Coverage increased (+0.4%) to 67.951% when pulling 11c837a on eyal0:backtrack into 4ad5465 on pcb2gcode:master.

[eyal0]

Blossom V requires undirected edges as far as I can tell so it won't suit.

As a test, after enabling backtrack for all of the integration tests, I had pcb2gcode output all the segments that it's going to mill. The only difference with/without backtrack is that there are segments that get milled more than once. However, there are no new segments being milled. Also, the direction of segments that have a required direction is maintained. So that's convincing.