Consolidate secondary vertices

This MR changes how secondary vertices are stored. Successfully fit SVs are now stored back-to-back. The SV index no longer encodes any information about the track indices. The track indices are instead stored in the SV struct. This eliminates the need to allocate a SV for every possible track combination. Additionally only SVs that pass track preselections are looped over in subsequent steps, leading to a modest speedup (72.1 kHz vs 71.5 kHz on Titan V).

This will require substantial changes in !240 (closed). This will also be the starting point for development of 3- and 4-track lines.