[hdf5tomoscan] improvement to handle different version of NXtomo (from mxtomomill)

• add nexus_path and handling of nexus_version properties
• add missing getters
  • nexus_version
  • source attribute / class
    • contains name
    • contains type
  • instrument_name
  • definition ??? don't see the interest to have it here. I don't think we expect to have some else than NXtomo for now at our level