Add `simplex_downhill` and fix `find_best_polycomp_parameters`
Add `simplex_downhill` and fix `find_best_polycomp_parameters`

The optimization code has been considerably extended. The function
`find_best_polycomp_parameters` uses a less naive algorithm to decide where to
head in its hunt for a minimum. A new function, `simplex_downhill`, has been
added as well. It implements the Nelder-Mead method (described here:
https://en.wikipedia.org/wiki/Nelder%E2%80%93Mead_method).

ziotom78/polycomp