In SelectorCriterion & SelectorCriterions, when selecting a Net occurrence,
we where storing the Net only. This was fine if the Net was belonging to
the Cell's top level. But when it was an occurrence of a non-top level
net, this was creating the elusive incoherent Occurrence problem.
Now we truly store the occurrence of the Net, to be accurate, the root
of the HyperNet.
* Change: In SelectorCriterions::add(), the Net* argument is replaced
by a Occurrence of Net. Must be an HyperNet root occurrence.
Same goes for SelectorCriterions::remove().
* Change: In CellWidget::select(), when called with a Net occurrence,
select the whole HyperNet components.
* Change: In NetSelectorCriterion, now use a Net occurrence instead
of directly a Net. Must be an HyperNet root net occurrence.
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