Fix compilation with Pythia 8.304

Summary: Pythia 8.304 switches to shared pointers (from raw) for ParticleDataEntry returns. Introduce version-dependent typedef to deal with this.

Test Plan: Compiled successfully with 8.304, 8.303, 8.244

Reviewers: jback, kreps

Reviewed By: kreps

Maniphest Tasks: T144

Differential Revision: https://phab.hepforge.org/D68