WIP: iterative reweighted MxNE with BCD solving MxNE, added depth weighting proposed by Haufe et al.

This pull request adds several functions to the inverse_sparse module:

- BCD code for computing MxNE
- iterative reweighted MxNE for solving L0.5/L2 mixed-norm problem
- depth weighting proposed by Haufe et al. based on source variance estimate
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> Average EEG reference is present in both, evoked and cov. The residuum in EEG seems to have a different reason.
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the fact that the residual of EEG is clearly not zero mean is suspicious.
Do you see the same pb with other conditions?