Preparation for new mixing part 8

Same as pull request 2016, plus a modification to give meaningful names (instead of numbers) to the mixing options.
The default is still the old mixing.
The issue pointed out by @Dr15Jones (about which TrackingRecHits are accessed) is not solved yet, but I'd like to get this pull request merged into a release as soon as possible anyway, to prevent my code from further diverging from CMSSW. It only affects the new mixing (not default), and the code compiles and runs even so.