Add support for Sherpa 3

Summary:
Adapted Sherpa interface to support releases 2 and 3

Update CMake modules for Sherpa 3

Move CI to LCG_106a to pick up Sherpa 3.0.0

Modify CI to use Sherpa 2 for legacy builds

Implemented modification to use PHOTONS::AddRadiation function directly.

Removed unnecessary setting of blob status

Introduced finalise function inside EvtSherpaPhotons class to control the order of static destruction.

Updated setupEvtGen script for Sherpa 3

This is related with !39 and T250.

Test Plan: Run all FSR tests for the three supported Sherpa options and check that the results are the same as with the master branch irrespective of Sherpa version.

Reviewers: tlatham, kreps, jback

Reviewed By: jback

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D134