MRG: add ems filtering

This adds the EMS filtering functionality described in[1]

todo:
- add proper tests
- more convenient API , maybe instance method for epochs
- factorization + parallelization
- add tested support for other objective function
- add covariance (optional)

# References
   [1] Aaron Schurger, Sebastien Marti, and Stanislas Dehaene, "Reducing
        multi-sensor data to a single time course that reveals experimental
        effects", BMC Neuroscience 2013, 14:122