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 
This offset appears in exactly the same form three times. Can you make it a simple function of bx or at least define "7-3" once somewhere (could you elaborate for me where is the transition -3 -> 7-3 coming from? having to do with time slots I guess  but still not clear why the result).