Add functionality to get size of the configuration space

[mfeurer]

The function should return np.inf if at least one float hyperparameter is in the configuration space, otherwise it should count the number of possible configurations.

[mfeurer]

Related to #142 which generates a grid of all legal configurations. This could be used as an alternative, although it might be infeasible to generate all configurations. That function could also be converted to a generator so that we can make that function more useful.