Herwig721 library loading order issue

As described in AGENE-1877, it is observed that the order of these libraries causes a problem in Herwig721 where the LesHouches class does not have any of the required functionality. This has been verified using Herwig721 interactively, and is fixed by setting up the LHEHandler before the LHEReader. Further study may be required but in the current format, Powheg+Herwig721 is not functioning.

Tagging @dhirsch as he is also looking into this and the library inclusion.