ENH/BUG: unify rank computaiton and add better support for SSS filtered data

@agramfort @Eric89GXL @mluessi FYI
It's not applicable to cov unless estimated on row data. We should keep it for backwards compat.

> On 26 Dec 2014, at 12:40, Alexandre Gramfort <notifications@github.com> wrote:
> 
> In mne/utils.py:
> 
> > @@ -323,6 +326,13 @@ def estimate_rank(data, tol=1e-4, return_singular=False,
> >          return rank
> >  
> >  
> > +def _compute_row_norms(data):
> In what situation row scaling does not yield the same rank as scalar scalings?
> 
> —
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