Add surrogate generation from evoked data

[tsbinns]

Fixes #251

• Adds make_surrogate_evoked_data for generating surrogates from evoked data.
• Updates the example to demonstrate the new function.
• Adds make_surrogate_resting_data which will replace make_surrogate_data in a later release.
• Adds support for Epochs objects to make_surrogate_resting_data.

Main thing missing is a test that validates the make_surrogate_evoked_data approach. Our existing data simulation only generates pseudo-resting data, but we need to be testing on evoked data.

We could use something from an example dataset (e.g., somato - like we do for the example). We could also look at adding functionality for simulating evoked data, but that's a much more considerable undertaking and I'm not sure how much use there would be for it outside of this.

Will leave in draft until the testing situation is clearer.