Reduce the gcell storage

[maliberty]

Using #4615 I see that the gcell data takes a lot of space in the .odb file. The current structure is:

  struct GCellData
  {
    uint horizontal_usage = 0;
    uint vertical_usage = 0;
    uint up_usage = 0;
    uint horizontal_capacity = 0;
    uint vertical_capacity = 0;
    uint up_capacity = 0;
    uint horizontal_blockage = 0;
    uint vertical_blockage = 0;
    uint up_blockage = 0;
  };

This is 9 4-byte ints which is a lot of space. A large design can easily have a matrix with 10M entries for each of 10 routing layers.

Oddly we don't actually write out the blockage fields:

dbOStream& operator<<(dbOStream& stream, const dbGCellGrid::GCellData& obj)
{
  stream << obj.horizontal_usage;
  stream << obj.vertical_usage;
  stream << obj.up_usage;
  stream << obj.horizontal_capacity;
  stream << obj.vertical_capacity;
  stream << obj.up_capacity;
  return stream;
}

which seems like a bug. So the on disk size is a smaller than the in memory size. I think there are two ways to reduce the size:

1. Use uint8_5 (1-byte) as you generally have ~15 tracks per gcell so four bytes is wasteful
2. Reduce horizontal/vertical to just preferred. There is no need to store the non-preferred direction values as the global router will never route in the non-preferred direction.

A third possibility is to drop 'blockage'. This is only used in one spot in gpl's routability. Can we just reduce the capacity accordingly and check for low capacity gcells instead? (@eder-matheus ?)

[maliberty]

In bp_single/gf12 the gcell data is 2011.7MB of 3615.1MB = 55% of the total size