Const fix in CondFormats/ESObjects

I made both `operator[]` operators to throw if it's not `EcalPreshower`. That resolves two FIXME items and currently passes a short matrix (testing full matrix). If you look into `ESCondObjectContainer` it raises a question. All operations on it checks for `EcalPreshower`, thus maybe design of container is wrong (C++ static type checking is not utilized). Actually both `operator[]` and `find` should never even check for `EcalPreshower`. I assume class contract states that only `EcalPreshower` can be in the container. Check `insert`.

I would suggest to drop checks, because `insert` already guarantees all are  `EcalPreshower`.

I will update after a feedback.

Regarding `struct ESPedestal`. You don't need getter methods, it's a struct, all members are public by default.