Add Levenberg-Marquardt optimiser (requires sensitivities)

[MichaelClerx]

As a good example of a very simple / classical optimiser that requires sensitivities

https://en.wikipedia.org/wiki/Levenberg%E2%80%93Marquardt_algorithm

[MichaelClerx]

Lourakis (2005) A brief description of the Levenberg-Marquardt algorithm implemented by levmar