Change GUI default parameters to make 2D layouts more dilute

By default, particles in a layer are uncorrelated (interference function = none). This is physically meaningful only if the particles are dilute.

When we construct a particle layout using the current default parameter values proposed by the GUI, then the particles are far too dense. In some cases (e.g. dodecahedra), they are all interpenetrating.

We should reduce the default particle density, and make sure all particles have the same radial extension.