extFindNodes() is called to find just the substrate node, then
ExtLabelRegions() is called to label the substrate node if such a label
exists. The ExtLabelRegions() routine has a section at line 344 in which
if it finds a label on the substrate plane that is over space, then it
attaches the label to the default substrate region. However, if the
label is inside an isolated substrate region, then it passes this check,
which never actually detects whether or not the label is over space.
Doing so is actually simple, since the preceding code detected connecting
tiles under the label, and that fact just needed to be carried forward
and checked. The upshot of the error was that if an isolated substrate
region like a pwell inside deep nwell was *labeled*, then it would
mysteriously get shorted to the global substrate, which is clearly wrong.
Thanks to Mark Martin for providing the failing example (and apologies to
Mark Martin for spending time trying to debug a very obscure problem).
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