Hybrid Pressure load and save support for PP

This PR adds support for loading and saving PP files with a hybrid pressure coordinate as indicated by an lbvc value of 9. This is a big PR resulting from three people working in collaboration and I'll be happy to split it up if necessary.

Remaining tasks:

- [ ] Determine appropriate value of ```PP_HYBRID_COORDINATE_REFERENCE_PRESSURE```. I've currently set this to unity.
- [ ] Fix copyright dates