Check initialization of variables and use consistent mechanisms

[pausz]

Member functions Class::init and Class::xinit, where xinit is phinit in the case of Propagator and nuinit in the case of Coupling, use different mechanisms to access the same information (eg, the initial value of the variable).

The mechanisms should be the same and /or have consistent behaviour.
Fixing this would avoid issues or difficulties in finding bugs like the ones explained #101, and the one describes as a NOTE in #160.