Nelder-Mead downhill simplex optimisation

Nelder-Mead simplex optimiser, according to Saša Singer and John Nelder (2009), Scholarpedia, 4(7):2928. doi:10.4249/scholarpedia.2928

Implementation for functions returning real values from vectors of reals.

This patch does all the heavy-lifting for rigid-body-fitting structures to densities.

Resovles last part of #3581