Make ESPedestal::zero a const

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Make operator[] operators throw TypeNotEcalPreshower exception

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Drop all checks for EcalPreshower, container guarantees all items are EcalPreshower

Signed-off-by: David Abdurachmanov <David.Abdurachmanov@cern.ch>
Merge pull request #1533 from davidlt/const-fix-CondFormats-ESObjects

Multithreading fixes -- Const fix in CondFormats/ESObjects