NCrystal_process use obsolete packing_factor parameter.

[tkittel]

@mads-bertelsen I noticed that the NCrystal_process for Union still has the packing_factor that was originally in NCrystal_sample.comp but was removed when the density became configurable in the NCrystal cfg-string itself.

So this parameter should be removed from NCrystal_process as well at some point. It is not urgent, but could cause some users to assume they can use this parameter, in which case I am not sure what would happen (i.e. if results would be consistent with that packing factor or not).