Problem with nDim with 2D potentials

[linkerst]

Using a 2D potential currently creates in error in the ##set dim part of basic_systems. This is due to the sympy representation of potential.nDim. Fix when sympy variables or corresponding valuesare used.

[RiesBen]

I fixed the issue by generating a constants dict for all potential, which is a good solution, as this is needed for the sympy implementation style. And I would like in future to use this more.

Checkout the new Bas_simulations.ipynb - it contains a simple 2D case

see commit: #2

[RiesBen]

@linkerst: Please Test Langevine with the 2d Simulations in BasicSimulations