Add Fit performance test

[Anders-Markvardsen]

Add fit performance test(s)

[FedeMPouzols]

The initial plan would be to add tests that play the role of good examples for others willing to add more tests.

Ideally minimum coverage should be:

• a couple of simple input datasets: one 'peaks' example and one 'smoother but interesting shape function' example
• a bunch of different functions or at least a couple of very different ones
• some variant, like for example tests with constraints