improve support for 3D structures, and review particle reprocessing

• Correct 3D intensity by thickness factor
• Revert code duplication introduced by ad-hoc support for Liquid3D
• abundance vs admixture - it's too complicated
• Sure that surface density isn't taken into account twice?
• admixedFraction contains factor 1/thickness, unless thickness=infty; this looks inconsistent
• Document 3D random structures
• Extend to 3D lattices