AmpGen / LbAmpGen cleanup

Cleanup of (Lb)AmpGen: Removed the AmpGen that is stored in Gauss, instead following what is done for EvtGen and building from a tag on the GitHub. Still to do:

• The exact mechanism for (re)creating the source needs a bit more thought. At the moment, its via a CMake flag to a test directory just to see if the basic mechanism works. The best might be a commit hook that works in the CI if-and-only-if the source for a particular model doesn't already exist (or if the tag changes) - rather than have users explicitly commit the code for the models.
• Contents of LbAmpGen (options/src) is still from the master - should merge in the new options - with src created by whichever mechanism we settle on from the first item.
• The master from AmpGen's github is being pulled and built at the moment - will switch to an explicit tag that we make at the end of the review; some minor changes from the tag (v2.0) that exactly reproduces Marian's changes in the Sim09 branch to make the build play nicely inside the LHCb stack.