* Bug: In cumulus/plugins.block.block.py, always import Python modules
using the exact same path. Otherwise the module may get imported
twice and static variables are duplicated, generating a big mess.
This was causing problem for the LUT in macro.py, and got SRAMs
blocks encapsulated twice.
* New: In cumulus/plugins.block.block.Block.addPlaceHolder(), create
a "place holder" instance over a given area to prevent the placer
from using it. Allow to make space reservation.
* New: In cumulus/plugins.block.configuration.BlockConf, copy the
toXPitch() and toYSlice() methods from spare in order to share
them between modules. Still have to remove some other local copies.
* New: In cumulus/plugins.block.spare.QuadTree, keep a list of all
the X centers of the partitionned areas. For yse by the power lines.
* New: In cumulus/plugins.chip.chip, move doPowerLayout() call from
doChipFloorplan() to doConnectCore(), this is to delay the call
until *after* the spare QuadTree has been created and we can
align the power lines to the centers of the QuadTree.
* New: In cumulus/plugins.chip.pads.Corona.doPowerLayout(), if a
spare QuadTree has been created, align the power lines on the
X center of the leaf areas. This is a cheap way to avoid DRC
errors between the power BigVias and the wires from the various
clock trees (on METAL5).
* New: In cumulus/plugins.block.macro, add an ad-hoc patch for Staf's
SRAMs. The blockage areas are slightly too narrow. We enlarge
them by one pitch.