Add non-const getter for Collections in EventStore

[vvolkl]

Fixes #94

BEGINRELEASENOTES

• add non-const getter for Collections in EventStore

ENDRELEASENOTES

[gaede]

would it maybe be nicer to add the following method instead:

/// access to non-const pointer that allows modifying the collection
template<typename T>
T* EventStore::get(const std::string& name) 

this requires less typing ...

[hegner]

I find such syntax quite dangerous. If mixing a const T& and T* interface of the otherwise same signature, I can see sloppiness and usage of the T* version raise drastically. Which is bad for thread-safety.
Anyhow, I just realised that somehow the const-handling of the interface does not seem consistent at other places. Should we make that a discussion topic?

[gaede]

As discussed in the EDM4hep meeting, we postpone this PR until we have a better understanding of the needed semantics of the EventStore API.