Add text file serialization to meas_algorithms Defects class

It was noted on RFC-595 that the creation of the Defects text files to be stored in the data repositories is distinct from Defects. It would be more useful for people creating defects lists if the Defects class could serialize to the text format.

Robert Lupton also requests that metadata be stored in the text files (at least the format version number and the detector name and instrument). To satisfy that requirement I will see if I can write out in the Astropy ECSV format.