v0.2-9

• vcov.PlackettLuce() works again for ref = NULL (bug introduced with vcov method in version 0.2-4)
• avoid dependency on R >= 3.6.0 by providing alternatives to asplit()
• read.soi() and read.toi() now handle incomplete rankings with very irregular lengths correctly.
• read.*() functions for Preflib formats now give a meaningful error when the file or URL does not exist, and a warning if the file is corrupt.
• as.rankings with input = "orderings" now checks coded values can be matched to item names, if provided.
• PlackettLuce() now works with nspeudo > 0 when there are no observed paired comparisons.
• ?PlackettLuce now gives advice on analysing data with higher order ties.