Initialise More Data Members at Construction Time

In particular, apply explicit default constructors to most data
members and push initialisation to initialiser list if convenient.

While here, also split long lines and apply const in more places.
Finally, reset well- and connection-level PI values to zero in
WellState::shutWell().  This is in preparation of including shut
wells in BlackoilWellModel's internal state.

opm/simulators/wells/WellInterface_impl.hpp

@@ -271,7 +271,7 @@ namespace Opm
     WellInterface<TypeTag>::
     phaseUsage() const
     {
-        assert(phase_usage_);
+        assert(phase_usage_ != nullptr);
 
         return *phase_usage_;
     }