Implement a `brownian_motion!` function

[mastrof]

This function would simply move the agent randomly, with diffusivity defined by the Stokes-Einstein relationship $$D = \dfrac{kT}{6\pi\eta r}$$ Properties of the medium (temperature and viscosity) can be added to the DEFAULT_ABM_PROPERTIES, defaulting to the values for water. Then brownian_motion!(microbe, model) would just use model.viscosity, model.temperature and microbe.radius to generate a random step.

[mastrof]

This is probably worth doing after #78 is done