grade.pl, loa.pl, test_span.pl: Included consistency check (number of arguments) from the program call.
sample2.dat: Changed a few answers and misreadings from optical reader.
grade.pl: Seems to do the complete job now, that is, reads the optical
	  reader file from a class, searches for test kind of each
	  student, grade its test, appends the scores to the
	  corresponding class .dat file and, finally, uses it to create
	  an HTML file with students' scores.

	  It would be desirable, after at least three tests are done,
	  the program to append an extra column with the final score of
	  each student.