* In CRL::GdsStream::_staticInit(), all the layers where added to the
translation table, whether or not they where configured for the
GDS stream. So the non GDS configured layers got a GDS layer
id of 0 and were using this case. Either overwriting the legit
layer or creating a new one while it should have been invalid.
Now we check for the hasGds() predicate of the layers.
* In CRL::GdsStream, add a new option to tell that, layer id 0,
if undefined, may be used as the definition of the boudary of
the cell (abutment box).
* In CRL::PyGds, now also export the flags to the Python interface.
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