implement PREP pipeline

[bnicenboim]

PREP pipeline, which automatically and reproducibly a) removes line noise without applying a notch filter, using a sliding-window algorithm, b) performs robust average re-referencing on the data using a RANSAC-based algorithm to detect and ignore noisy/otherwise "bad" channels while calculating an average reference, and c) optionally reinterpolates channels still flagged as "bad" after referencing using data from the non-bad electrodes and a 3D spatial model of the good electrode locations. There's a WIP Python-based version by one of the MNE mantainers here

Originally posted by @a-hurst in #144 (comment)