A cofficient in `PSW_func` should be `168`

[ljw20180420]

I think this line

polychrom-hoomd/polychrom_hoomd/forces.py

Line 39 in 27b983b

* (168*(r_max - r_min)**2 - 120*(r_max + r_min - 2*r)**2) \

should be

* (168*(r_max - r_min)**2 - 168*(r_max + r_min - 2*r)**2)  \

I found this by comparing it with poly_func.

polychrom-hoomd/polychrom_hoomd/forces.py

Line 24 in 27b983b

F = epsilon * r**11 * 84 * (cutoff**2 - r**2) / cutoff**14