Speed improvements for ECAL multifit

Roughly 30% faster in offline mode on a Xeon X5650. 
There is probably room for more speedup but it will require substantial work.
Not a factor of 10 but it's something.

Main sources of improvement are:
- turning off debug for Eigen :-)
- removing use of eigen "views" that seems to not work. Should probably report to eigen people.

There are a few places where non-unrollable views are used because of the eigen interface, but that's life.

No differences expected/observed.

@VinInn @bendavid 
I will make my (already) usual comment that  eigen_solve_submatrix(..) and eigen_partial_cholesky_solve(...) can be implemented with an order of magnitude less lines and this will also avoid code repetition. Why not doing it that way?