Fit a noisy sine

You will fit a sine to 60 noisy readings of a swinging spring, with a first guess from the FFT.

The idea. The FFT finds the main frequency to within one bar. curve_fit then refines the amplitude, the frequency and the phase, and pcov gives the errors.

Your tasks.

1. Write model(t, A, f, phi): the sine $A\sin(2\pi f t + \varphi)$.
2. Set guess to the frequency of the tallest bar of the FFT, in Hz.
3. Fit with curve_fit, starting from [1.0, guess, 0.0]. Set f_fit to the fitted frequency and f_err to its error.