[fluxvars] Make FluxVariables default constructable

This commit contains backward imcompatible. All flux variables
now have to be default constructed. Implementers of flux variables
need to make their implementation default construcable too.
All calls to the old non-default ctors will throw errors at runtime and
emit deprecation warning at compile time. FluxVars are no constructed
and update like this:

FluxVariables fluxVars;
fluxVars.update(...)

Being default contructable offers more flexible class inheritance
and more flexible caching possiblities.

To this end especially the 2pnc/2pncmin flux variables where
updated to comform with other models. This involves some
deprecations. The 2pncmin reference had to be altered in the
velocity. The velocity was previously outputted wrongly as
the correct volumeFlux variable was never computed for 2pnc/2pncmin.