Allow for precomputed quantum kernels in quantum kernel methods

[JanSchnabel]

• Solves Issue#144
• After consulting @FredericRapp decided to leave QGP methods as is

[rothma]

Why neglect QGP?

[JanSchnabel]

Why neglect QGP?

Since sklearn also neglects it :D This is a weak argument, I know. Thus, I can work on it and also allow for the "precomputed" option in QGPs

[JanSchnabel]

@FredericRapp , can you please check QGPR?

Since QGPC wraps to the respective sklearn method, which does not provide a "precomputed" option, I decided to neglect this