Introduce electric fields

[reubenjohn]

Create electric fields that work as a function of distance
Possible implementation:

• create a ELECTRIC_FIELD class
  • it could have about 3 vectors a,b,c which represent coefficients of a quadratic function of position
  • have a feild(vect<> position) that returns a vector corresponding to the electric field at that position based on the a,b,c constants