PowhegControl: change pwhg keyword to width_correction and fix LHE header conversion in PowhegControl lhe updater
This merge request includes these changes needed for running the inverse width correction with the new bb4l PWHG version:
- correct the PWHG keyword widthCorrection to width_correction
- add width_correction to the list of keys for which parallel XML reweighting is allowed (XML reweighting seems to be needed for the inverse width corrections to work)
- convert byte-string output of
ElementTree.tostring(header_new)
in lhe_nominal_weight_updater.py to a string such that the header is correctly written out in the final LHE output file (this was discussed in AGENE-2291)