Improve recovery semantics

[MLopez-Ibanez]

The semantics of recovery are not clear. Should it recover the exact parameters given to the original run? If so, then moving to a new execdir will mess up things. Should it allow changing parameters after recovery (that is, what to do if it finds a new scenario file with different values or it is passed additional command-line parameters?).

One possible behavior could be: Recover everything as given originally; if new settings are given, either by a configuration file or command-line, override them; for settings that are still set to default values in the original, recompute the default (because overridden settings may influence the default of other settings). Report everything that is restored and overridden.

This means recovery should happen much earlier, before reading command-line/scenario file.