Representation:
  In spatial simulations the physical quantities depend on position in space
  and can be represented by

fields - continuous distribution (scalar, vector, tensor)
particles - discrete distribution

  Transformation between the two representations:

   distributed particles -> particle density function   --> field f (r)
               field f (r) ------> sampling with density f(r) -> particle representation

  Processes:
  process can be modeled as

• evolution of fields
• evolution of spatially distributed particles