Added slicemethod to genFileSet.
The slicemethod is designed to have similar outcomes to Fit2d's 'File Series' methods. This produces a set of files which are, within the start stop bounds, and an integer mulitple of step from one another.  The structure could be improved by using a start and stop file and parse the start and stop file numbers from there.  Additionally, this assumes that the defining number is at the end of the file.  Finally, this only handles integer numbers, no more complex numbering schemes.