21.0 PixelPrdAssoication improvements

Hi,

This merge request implements improvements for the PixelPrdAssoication configuration:

1. Extends the minimum number of hits to 3 to improve reconstruction efficiency for shorter tracklets
2. Enables an SCT hit filter to improve reconstruction efficiency, SCT hit veto efficiency, and purity of selected tracklets
3. Fixes a bug in SiTools_xk (in regards to the SCT hit filter)
4. Uses PixelPrdAssoication configuration for vertexing code which previously relied on the PixelThreeLayer configuration.

Since (1)-(2) these changes would violate the frozen T0 policy, they were protected under a flag (default false) so that the original configuration in 21.0 remains, as recommended from the reconstruction SW experts. These changes will be included in a RPVLL reprocessing of 2018 data.

Cheers, Matthew

Tagging @mhodgkin, @goetz , @mdanning , @npetters