feat: spectrum interpolation

[drammock]

Carrying this over from the (now-closed) MNE-Python issue

Permission to translate the FieldTrip code was granted here: mne-tools/mne-python#7170 (comment)

• FieldTrip implementation: https://github.com/fieldtrip/fieldtrip/blob/master/preproc/ft_preproc_dftfilter.m?rgh-link-date=2019-12-31T04%3A11%3A36.000Z#L147 (link may be out of date / point to incorrect line)
• Reference: Leske, S., & Dalal, S. S. (2019). Reducing power line noise in EEG and MEG data via spectrum interpolation. NeuroImage, 189, 763-776