* New: In cumulus/plugins.block.hfns4.py, perform simple HFNS by breaking
the net into sub-nets of at most 10 sinks (hard-coded for now).
As this method is called *after* the netlist as been virtually
flattened, we have to create the RoutingPad at the top level
ourselves. Sub-nets are created at the Cell top-level (same
approach as for clock synthesis, because there is no smart way
to guess where they should be).
* New: In cumulus/plugins.block.block.py, perform HFNS (#4) *before*
doing placement. To see the real sink count on each net, we must
perform the virtual net flattening first (Cell::flattenNets()).
* Change: In cumulus/plugins.block.configuration, allow the creation
of spare buffer in any cell (instead of only "self.cellPnR").
* Change: In cumulus/plugins.block.spares.Spares.raddTransNet(),
Check if intermediate masterNet exists in Cells before trying
to blindly re-create it.
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