D1: add a scaling factor to all potentials

[bettinakeller]

We would like the option to change the relative weights of the potentials in a combined potential. Example applications are free-energy perturbation

$$\lambda V_1(x) + (1-\lambda)V_2$$

or optimal control

$$ V_1(x) + \lambda(t) kx$$

I think, the best way to implement this is to add a scaling factor $\lambda$ to each potential class whose default value is 1.

• document in manual
• implement in potentials/D1.py
• update benchmarks for the D1 potentials / test