This internal function obtains data from different sources and formats them as data frames
for use in other functions.
If dat.in is already a data frame in R, the data frame is returned.
No other types of R objects are permitted.
If data are stored in a tab-delimited file in the working directory, dat.in
references the filename as a character string enclosed in quotes.
The tab-delimited file must be readable using
read.table(filename, header=TRUE, sep="\t").