Add quality overflow to search node score

[KonaeAkira]

The score would be (most important metric first):

• Quality
• Duration
• Steps
• Quality overflow

This would improve situations like this (v0.9.0):

In the example, Innovation can be moved 1 step to the left, increasing quality overflow.

[KonaeAkira]

Things to consider:

• Does state.missing_quality need to be replaced with state.quality?
• Does including quality overflow in a search node's score increase time to solve / memory usage?

[KonaeAkira]

Solved with #57