Reading the proton gradient

A student measured the pH inside and outside the thylakoids of three samples. You will find the steepness of the gradient in each, say which side is more acid, and name the sample with the steepest gradient.

The idea. The samples are in a dictionary, as pairs of the lumen pH and the stroma pH. Each pH unit is a tenfold change in the proton concentration.

Your tasks.

1. In fold, return how many times higher the proton concentration is in the lumen than in the stroma.
2. In acid_side, return "lumen" when the lumen pH is lower than the stroma pH, and "stroma" otherwise.
3. Set steepest to the name of the sample whose gradient is the steepest.