reading data from Excel files

[Yugal0]

There is a feature (in preprocessing named "readCSV") to take impedance data stored in a CSV file, but there is not such functionality for excel files (.xls or .xlsx)
Many people have their experimental data in excel files as well.
It will be a short implementation, I guess, if any dev is interested, please do so. I myself would like to do but new to GitHub etc., so there is some friction.