Issues/364 prevent reads shuffle

[ducku]

Reads outside of nodes are sorted by order, then a previous y value. However, consecutive reads outside of nodes have their y values computed simultaneously, resulting in the previous y value still being null. This resulted in consecutive read segments outside of nodes that have the same order value being randomly sorted.

Fix: Go back through the path and find the last computed y value(from the last visited node), and use that to sort segments.

Closes #364

chr6:82003584-82004084

[adamnovak]

This looks good, I am just going to slightly tinker with the logic.