Long term feature request: Multiple input data sets

[billdenney]

It would help to be able to merge many datasets. I often get data as concentration in one file, dose in another file, demographics in another file.

Another use case is to provide a mapping of one set of values to another set of values. (I get the raw data with VISIT but no VISITDY, I'd like to make a mapping from VISIT to VISITDY. I may manually type in the mapping or use a regular expression.)