[post Nifty5] make Fields and MultiFields immutable?
MultiFields are the only NIFTy classes whose value can change after they have been constructed. It may be worthwhile to change this and make them immutable as well.
- code will get quite a bit shorter (e.g. no more explicit
- less potential for subtle usage errors
- slight performance hit by additional copying of data (operators like += will not be as efficient any more)