- add copy assignement to MaterialData

Initialise field values, this fixes an issue in MarlinTrk where wrong magnetic fields were assigned to surfaces because the field array was uninitialised

  - Improve conditions handling. Started to implement using simple telescope
    detector from C.Burr.
  - Add variable DD4HEP_LIBRARY_PATH to be used on MacOSX El Capitane
    to overcome DYLD_LIBRARY_PATH protection issues.
  - Fix to copy with negative(rebased) IDDDescriptor fields
    (affects IDDDescriptor and VolumeManager).
    2 New tests:
    - ClientTests/Bitfield_SidesTest.xml to test readout string:
      system:8,barrel:-2
    - ClientTests/Bitfield_SidesTest2.xml to test readout string:
      system:16,barrel:16:-5