External Fields support

[InnocentBug]

PySAGES should support a way to support external fields into the simulation.

We are envisioning a separate API that supports this. The external field should be passed separately to the pysages.run function to allow external field in tandem with each method. The external field gets an API that allows the definition of potentials via functions and tabulated fields.

Optimally, we are thinking about time dependent potentials as well.