Ecal multifit optimized

Move to Eigen for vectors/matrices, remove std::set, and some optimizations of the algorithm itself (avoid explicit matrix inversion and make some of required matrix solve operations less complex)

Didn't manage to run igprof yet, but based on a test of an earlier eigen version which @VinInn  did there shouldn't be anything very bad here.

Output should be strictly identical down to numerical precision.  (Verified on one event that all rechits are the same down to 5 decimal places, but there can be smaller numerical differences clearly.)

Speedup of the EcalUncalibRecHitMultiFitAlgo is just short of a factor of 3.  (~800ms -> ~280ms for pu40bx25 photon gun)

The EcalUncalibRecHitProducer has also been made a stream module.

@emanueledimarco @argiro @lgray 
And the "7" here and 10 lines above has to do with the "7-3" before so it'd better be a const to be used.