refactor: change colnames of chromatogram() data.frame

[jorainer]

• Change the column names of the data.frame returned by chromatogram() to "rtime" and "intensity" (issue Changing column names of chromatogram data.frames #304).

[jorainer]

Added a quick additional change - chromatograms() was (silently) changing the result object type: is a single chromatogram was requested it returned a data.frame, if no chromatogram was specified, or multiple were requested it returned a list of data.frame. This causes some issues and additional checks in the Chromatograms package. I added now a new parameter drop = TRUE to the chromatograms() function that can be set to drop = FALSE so that always a list is returned (even for a single data.frame).