Class to manage SAND subdetectors geometrical info

The idea is to introduce:

• a class describing a STT tube:
  • the position of the wire
  • the unique id
  • the path in TGeometry
  • the length
• a class describing a ECAL cell:
  • the position of the center of the cell
  • the unique id
  • the path in the TGeometry
  • the length
• a class storing the SAND subdetectors geometrical info:
  • a map: [path: cell]
  • a map: [path: tube]