[Mne_analysis] remove heat beats artifacts using SSP

[Alexandre Gramfort]

> However, I got an error because I calculated SSP separately for different
> runs, and the error is caused by the different projections obtained from
> different runs.

you should then compute the SSP on the concatenated raw files
and apply them on your Epochs or better recompute the Epochs
from the concatenated raws.

HTH
Alex