Add Multi-Emulator

[JBorrow]

Ever wanted your emulator to have more emulator? Well look no further, now we have put emulator's in your emulators so you can emulate complicated things.

This new emulator type builds upon the GaussianProcessEmulator class and trains separate sub-emulators for different independent variable ranges. Hopefully, this will allow us to capture the break in the mass functions better.

I've tagged you @Moyoxkit because I think you may be interested. I'll follow up with more information once I've tried running it on my 50 Mpc boxes.

[JBorrow]

Okay, so here's what the range of emulator outputs looks like when you train a sole emulator:

And here's what it looks like when you train three:

Obviously none of these are a good fit to the data but that's what we expect. Leave one out comparisons are coming.

[Moyoxkit]

Great stuff!