pyInterface: actually call a function in genpw.py

If the option '--randomBlockBeam' was present, the method
readBeamfileSequentially of the generatorManager was never called as a
pair of parenthesis was missing. This caused the error message
'randomizing beamfile starting position without sequential beamfile
reading has no effect', and the option was effectively ignored.

(cherry picked from commit 0005e8bbc837530ba96d5e7f87895ee54a4fc09a)