added _null_observation flag for observations that start with '!', corrected error when mag string is 0 length or set as float, changed property setter on coordinates to be Python3 compatible.
merged in difference from master to ensure match
Merge remote-tracking branch 'remotes/origin/master' into mpc
added 'comment' to mpc_line and create a to_line method to print out full 'OSSOS-style' mpc formated lines. Also renamed from_line to from_string to be clearer.