Added 'clean' to Makefile

It is customary to have a 'clean' option in the Makefile to remove files that are created during make.  We don't want to accidentally use outdated data files so we create a method to clean out the directory before re-running analyses.  Try running make clean and see what happens to the data directory.  It migt be wiser to move these files to a trash directory instead of removing them altogether.  We can move in that direction later if desired.