Alternative names for D-score and sem estimates in the logit scale

[stefvanbuuren]

Separate issues from #46

Currently both metric = "dscore" (default) and metric = "logit" create a data frame with names d, sem and daz. Depending on the metric option, the columns called d can contain either a score in the D-score scale (usually positive values) or in the logit scale (both positive and negative values). It would be better to use different column names for these, to avoid confusion. The same holds for sem. The daz in the logit scale has no meaning and should be removed.

Originally posted by @stefvanbuuren in #46 (comment)