Adaption of kernel optimizer

[DennisKleinhans]

• The KernelOptimizer extends the KernelMatrixBase so that it can be used as a kernel in the high-level methods.
• The KernelOptimizer now accepts a KernelMatrix, a loss function and an optimizer.
• The KernelOptimizer automatically optimizes the provided KernelMatrix when the fit method is called by checking the is_trainable property.

closes #274

[David-Kreplin]

I did some adjustments, maybe you can check them @MoritzWillmann

[David-Kreplin]

I’ll take it from here. Most of the work looks great, but there are a few specifics that are a bit tedious to explain and fix, such as regularization.