parameter system: defer first-time retrieval of parameter values to the end of the registration

this makes the usage message useful if something goes wrong when
retrieving a parameter.

Also, fix the output message if something goes wrong during grid
creation.
PVS model: don't force the phase presence to be converged

this partially fixes the SPE9 problem for the PVS model (at least it
can now simulate a few hours instead of nothing at all). The remaining
break-downs are probably intrinsic to the PVS model...

(For me, that's actually good news: Finally, the NCP model can
simulate something that the PVS one can't!)