New names in `mice` for lower and upper confidence interval

[stefvanbuuren]

mice 3.17.0 uses names "conf.low" and "conf.high" to refer to the lower and upper bounds of the confidence interval, and will drop the use of names like "2.5 %". This has the advantage that we can use the same name for any confidence interval. mice temporarily supports both name type, but will drop support of "2.5 %" names in the future.

Please update your code in your next release. Thanks.