Add support for other NLP solvers

[MatthewPeterKelly]

There are many NLP solvers that can interface with Matlab, potentially allowing users to try to solve problems using a core solver other than FMINCON. Two possible interfaces are with Knitro and SNOPT, links below:

Knitro: https://www.artelys.com/tools/knitro_doc/3_referenceManual/knitromatlabReference.html https://www.artelys.com/tools/knitro_doc/1_introduction/installation.html

SNOPT https://github.com/snopt/snopt-matlab https://ccom.ucsd.edu/~optimizers/downloads/index.php