Implement Sherpa's PHOTONS++ as alternative for final-state radiation

Summary:
This is a first full working version of an interface between EvtGen and
Sherpa's PHOTONS++ generator for final-state radiation.

This includes an additonal interface to use EvtGen's random-number generator inside Sherpa.

The intialisation of the interface updates Sherpa's KF_Table containing particle properties
based on EvtGen's PDL table.

The common model testing module was updated to add options for different FSR generators.
The possibility to specify the FSR generator as additional input argument was added to the testing module.

The modifications required to link with Sherpa's libraries and to build sherpa from source
where included in the respective EvtGen compilation files.

This is associated with T250 and !14.

Test Plan: Run compilation and new Sherpa PHOTONS++ tests.

Reviewers: kreps, tlatham, jback

Reviewed By: tlatham, jback

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D115