ENH: apply post-scaling to enable handling SSS rank when using grad and mag

[dengemann]

Currently the pipeline assumes that one sensor type is passed.
In practice, SSS is applied on both magnetometers and gradiometers and we should not require users to pick one of them.
The solution should be to apply post scaling inside the functions computing covariances (and other features) from epochs.
With post-scaling applied based on the MNE defaults, the ProjCommonSpace should be able to handle everything.
If SSS is not applied, different options may need to be elaborated.