Consistent API for fidelity functions

[lazyoracle]

Describe the bug

The fidelity functions could possibly be improved to have a consistent signature/API which would make their integration easier with the optimizer. Ref: https://github.com/q-optimize/c3/pull/136#pullrequestreview-791979783

To Reproduce

Expected behavior

Screenshots

Environment (please complete the following information)

Additional context

Related: #135