Add an interface for the PRIYA emulator

[andreufont]

The PRIYA emulator is not yet public, but it should be at some point: https://github.com/sbird/lya_emulator_full

It is not clear how to setup a LaCE-PRIYA emulator (the sims mix two resolutions), but we could imagine maybe using directly its PRIYA emulator. Not sure how much work this would take, since the parameterisation is very different. Maybe we could write some sort of wrapper that hides the details of the PRIYA emulator and provides and interface that is more similar to LaCE... but not sure how easy that is.