function depositParticles now takes optional argument to set approximation. Random2D::PY specifies PercusYevick 2D.

The APIs for uncorrelated-particle and hard-disk models were merged,

layer.depositParticle(density, particle, approx)

where approx=ba.Random2D_Dilute by default, or approx=ba.Random2D_PY` for Percus-Yevick.