Improve grid generation

[mfeurer]

The method added in #142 has two drawbacks that can become a limiting factor:

1. It generates all configurations at once. Instead it should be a generator.
2. It does not delegate generating the legal values of a hyperparameter to the respective hyperparameter class. This will make it not pick up on new hyperparameters which can be added.

[thijssnelleman]

@mfeurer Is this issue still applicable?

[mfeurer]

I think so.