To Do:
- Optimise scale/angle in multi_search responses
  - discard any poses which are not at a local minima in both
  - This seems to discard everything at present.  
  - Need to revisit.
- region_pdf_buider - memory overwrite somewhere?
- region_pdf_builder - add shape (eg box/ellipse) flag
- Localisation of minima when searching at multiple angles/scales
  could be improved by checking response images at nearby scales/angles
  - Would also reduce the number of false minima
  - A bit fiddly because evaluated regions don't overlap at edges
  - Implement sometime!

- Use mfpf_prune_overlaps in searcher class.
- Compute sensible limits and steps for the simplex optimisation
  in mfpf_point_finder::refine_match (depends on size of object
- radius() currently defined in model units - should it be in world
  units?
- Tune refine_match algorithm 
   - Probably using too many iterations at present.

- add switch for normalisation in region_pdf

- * Problem with vpdfl_pc_gaussian - not very sensible implementation
    (ie builds full square eigenvector matrix.  Fix this!)

