FPGATrackSim: implement ROOT output for second stage hit test vectors
We previously added a new branch to the output TTrees storing "sliced hits" that come out of the slicing engine and are the input to pattern recognition-- but only for the first stage. This commit does the same thing, adding a new branch to the second stage output tree for hits used in the "second stage" (extension/extrapolation), with one "tower" per slice, assuming that the number of slices is consistent between the first stage and second stage.
This has to be duplicated across the window extension and NN pathfinder tools, at least for the time being.
Note that at present in the inside out algorithm there will only ever be one slice here, both in the first stage and second stage. This is because the "slicing" part of the inside out algorithm is currently an internal piece of it; the actual "slicing engine" in this scheme is only doing regionalization (i.e. keeping hits that fall within the configured region and dropping the rest. In the future we may carve out the slicing piece of the inside out algorithm so it can be used to set slices for both the first and second stages, at which point this may change.
With this MR it should be possible to prepare ROOT files that contain the necessary information for second stage test vectors.