add meta data to Skeleton class

[huangziwei]

New

• added meta data to Skeleton dataclass, which includes
  • skeliner_version
  • skeletonized_at (datetime)
  • unit (default to nm as most mesh data are in nm)
  • id (segment id or name)

Change / Fix

• renamed soma.centre to soma.center...;
• fixed missing soma in plot.projection without mesh;