Corrections for univariate repo 8 8 19

MelbournePedestrian:

 cases with all missing removed
 value -1 replaced with "missing"
 arff and ts updated

Chinatown:

 two cases from test set each with a single missing value removed
 arff and ts updated

PLAID:

 comments changed to give correct meaning to class values
 final value of 0 on 2/3 cases removed as believed to be a formatting artifact.