Fitting growth curves

You will write two fitting functions: one that finds the doubling time of exponential counts with a straight line through $\ln N$, and one that finds $r$ and $K$ of logistic counts with curve_fit.

The idea. For exponential counts, the slope of $\ln N$ against $t$ is $r$, and the doubling time is $\dfrac{\ln 2}{r}$. For logistic counts, curve_fit searches for the $r$ and $K$ that make the sum of squared misses smallest.

Your tasks.

1. In doubling_time, fit a straight line to np.log(N) against t and return the doubling time.
2. In fit_logistic, use curve_fit to find r and K, with the start fixed at N[0]. Return them both.