"Performer" field missing

[proccaserra]

fix: add "Performer" field to isatools/isatab/validate/rules/rules_40xx.py @line 344.

not picked up by the testing suite (legacy)

typical validation error looks like this: {'message': 'Unrecognised header', 'supplemental': 'Unrecognised column heading Performer at column position 13 in table file s_ptx_test.txt', 'code': 4014}