ENH: sets are now searched for.

Some tools now output sets at the pointsInstance (if relating to geometry),
some at the facesInstance (if relating to topology).
So when loading them we now search for them from current time down
to facesInstance. They cannot be beyond facesInstance.
Adapted Time::findInstance to take optional limiting instance. Adapted
topoSet to use this. Adapted all uses of IOobjectList on sets to do the
same.