Custom Mean Function

[mclaughlin6464]

There is reason to believe that adding a custom mean function will drastically improve the emulator's performance. I want to be able to add

• [ ] The default of subtracting off a constant
• [ ] Adding a skl method that will fit the training data and subtract that out
• [ ] Letting the user pass in a custom one, like a linear theory model, that subscribes to some schema.

The first two are really easy, the last one is a little more difficult.