HardDisk follow-up

In !3157 (merged), 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.

TODO:

• Adapt GUI sample representation
• Replace class InterferenceHardDisk
• Visualize outcomes, compare dilute vs PY
• Update docs