Add a prefix to GDS sub-cell isolated in a "per cell" library.

The GDS file write all the cell names (SREF) in one namespace, so cells
with identical names, coming from different libraries may clash. When
a GDS is read *and* contains sub-cell, we isolate them in a sub-library,
but we must *also* give the cells in them unique names, we prefix by
the library name (which in turn is the top cell name).

crlcore/src/ccore/gds/GdsParser.cpp

@@ -810,6 +810,8 @@ namespace {
     if (cell) return cell;
 
     if (not Gds::getTopCellName().empty() and (cellName != Gds::getTopCellName())) {
+      cellName.insert( 0, "." );
+      cellName.insert( 0, Gds::getTopCellName() );
       workLibrary = _library->getLibrary( Gds::getTopCellName() );
       if (workLibrary) { 
         cell = workLibrary->getCell( cellName );