Support SBPLX optimizer from nlopt

[cpossel]

What should we add?

The SBPLX algorithm is an extension to Nelder-Mead and expected to yield better results.
Thus, it might be beneficial to add the interface to the NLopt implementation of SBPLX.

[cpossel]

Locally I made the changes already some while ago and feel like it would be better to share them here. So, I gladly volunteer to add the changes in the code.
Only question left: while doing so, should I also add the other NLopt local gradient-free algorithms? Should be very little work.

[cpossel]

See pull request #228