ENH: Speed up ECG and EOG detection / SSP calculation

I had some long raw files, and this sped up the calculation (and made filtering methods more consistent).

@mluessi, this sped it up mostly because ~90% of the time was spent in `_filter_attenuation()` in `mne.filter` because the signal was so long. Do you think we can do something like skip the filter attenuation tests for a sufficiently long filter...? This isn't the first time I've hit a hangup with that function being slow because the filter is long.