Maintenance of pot functions

A pot function, currently pot_tanh, pot_arctan, pot_erf and pot_sharp should have the following arguments:

1. positional r
2. positional R
3. optional parameter (exponent)

To ensure this, we could:

• Create an abstract class
• Define __call__() as @abstractmethod and @staticmethod.
• Implement several abstract classes (1 per pot function).

While we are there, we should also filter overflow warning.