What is the default particle density in ESyS-Particle simulations

Prior to revision 1032 of ESyS-Particle (Nov. 2011), there was some ambiguity with respect to the default particle density in ESyS-Particle simulations. The issue is discussed in this Answers thread:
https://answers.launchpad.net/esys-particle/+question/177401

From rev. 1032, the default particle density is equal to one model unit regardless of whether a particle assembly is constructed in-simulation or via an input geometry file (e.g. constructed using GenGeo). For assemblies comprised of variable-sized particles, the mass (M) of a particle of radius R is initially set to:
M = pi . R^2 in 2D simulations
M = 4 pi R^3 / 3 in 3D simulations

The ESyS-Particle developers discourage reliance upon default parameter values and have made efforts to minimise the number of parameters with prescribed default values. One is strongly encouraged to choose one's model units carefully and explicitly assign particle densities via a python call similar to this:

LsmMpi.setParticleDensity (
   tag = 12345,
   mask = -1,
   Density = 9876.54
)

This call will set the density of every particle with tag 12345 equal to 9876.54 model density units.

IMPORTANT: The call the setParticleDensity(...) needs to be made _after_ reading the geometry file, otherwise it will not affect the particles. Also, if the model is run in 2D mode, setParticleDensity needs to be called _after_ the force2dComputations(True) call to ensure the particle density is correcly calculated according to the 2D eqution above.