Make candidate_count enforcement less strict

[jduerholt]

This PR makes the candidate_count enforcement less strict:

• A warning instead of an error is raised if the strategy spits out more candidates than expected. This was also requested by @CompRhys
• In the FractionalFactorialStrategy also values despite None for candidate count are allowed, nevertheless it always returns the predefined number of candidates and ignore the value given for candidate_count.

[bertiqwerty]

Thanks. Agreed.

[bertiqwerty]

Thanks. Agreed.

Actually, I wanted to respond this to the other PR but it also fits here.