Fix a bug in two ks line

It seems that TwoKsLine is giving unstable behaviour in !655 (merged). This MR fixes an outstanding issue in the line which may solve that.

• Compare all tracks in the vertices (one such comparison was missing).

Separately, a couple of fixes are included:

• Use float numerals instead of doubles.
• threadIdx.y was being used - this would have led to a race condition, but luckily parallelism in lines only pan the X dimension. It now uses a sequential iteration instead.