Added missing const qualifiers in Path interface.

Change-Id: Ic919c7b87f567f4f270c2d723d4f1899b4c6a74b
Reviewed-on: http://gerrit.aldebaran.lan/47220
Reviewed-by: pdaouadi <pdaouadi@aldebaran-robotics.com>
Reviewed-by: jlamotte <jlamotte@aldebaran.com>
Approved: jlamotte <jlamotte@aldebaran.com>
Tested-by: jlamotte <jlamotte@aldebaran.com>
Added output stream operator to allow Path objects to be logged without explicit conversion to string.

Change-Id: Ic94e57607be602470f088e3648c0301e1b03555e
Reviewed-on: http://gerrit.aldebaran.lan/47245
Approved: jlamotte <jlamotte@aldebaran.com>
Reviewed-by: jlamotte <jlamotte@aldebaran.com>
Tested-by: jlamotte <jlamotte@aldebaran.com>