ENH: Added set/get methods for coordinate system in ImageMessage classes
BUG: LPS coordinate system value in ImageMessage does not match protocol

The COORDINATE_LPS enum value is 0 in the ImageMessage & ImageMessage2
classes, but the protocol says this should be 2. It is defined as 2 in
igtl_image.h (IGTL_IMAGE_COORD_LPS).

The enum has been changed so that the LPS value is 2.
BUG: merge labonny's changes