Bug: laser diffusor option treated as tuple

Looks like the Laser.diffusor argument is converted to tuple, so trying --Laser.diffusor = exp 5 option leads to diffusor type ('e','x','p') - which means it cannot be initialized.

This bug was discovered in work by Alexandra @rgufdgi, see the MR !44 (merged), where the test, reproducing this error, was added.