add support for HepMC 3.3.0
Adds support for HepMC 3.0.0
As discussed at the Rivet dev meeting:
- When using HepMC 3.3.x, we will use the HepMC3-intrinsic support for decompression.
- For HepMC 3.2.x, we keep using our current decompressor.
- Can be simplified if and when we drop support for 3.2.x.