This is the checking function design documented at
http://wiki.nmr-relax.com/Relax_source_design#The_check_.2A.28.29_functions. The
validate_sequence() function has been renamed to check_sequence_func() and the checking object is
called check_sequence. It removes the string processing hack to convert RelaxErrors to
RelaxWarnings in the lib.sequence.read_spin_data() function, avoiding strange messages such at
"RelaxWarning: ror: The sequence data in the line..." as seen in the
Sequence.test_bug_23372_read_csv system test.
