Point-cloud tracker improvements

[imagejan]

See improved-tracking branch.

• Return pruned graph (no links skipping over spots)
• Re-implement tracking using descriptor distance (instead of matching descriptors across entire field of view)
• Add setting to select between raw graph (with edge weights corresponding to model cost) and pruned graph
• Implement minCoverage filtering as defined in settings
• Implement max displacement constraint
• Clean up config panel
• Work on visible spots only (to allow spot filtering before tracking)