Support for exclusion of fragmentary witnesses

With this release, you can exclude fragmentary witnesses from your collation by specifying the --fragmentary-threshold command-line option, followed by a number between 0 and 1 indicating the proportion of variation units at which a witness must be extant (i.e., have a non-missing reading according to the reading type(s) specified with the -m option) to be included in the output. Thus, --fragmentary-threshold 0.7 will exclude all witnesses with more than 30 percent of their readings missing, while --fragmentary-threshold 1.0 will exclude witnesses with any missing readings. (Note that this check is performed after correctors' hands have been filled in, if you have supplied the --fill-correctors option.)