Convert Eprime Frames into data-frames
an EprimeFrame object, or a FrameList object (a list of EprimeFrames)
Individual EprimeFrames are converted to a data-frame using
as.data.frame. (Strings are not converted to factors.)
Each of the individual data-frames are then
rbinded together, with
missing columns being filled with NA.
all of the EprimeFrames combined into a single data frame.