- For some reason, gcc does not find the typedef Traits otherwise

For vectors, get_entry_ref is operator[], and the latter is preferred. For
matrices, a mutable reference to a single entry is not supported by all
backends and we make no use of it anyway.

-renamed get_entry_ref to get_unchecked_ref such that get_entry_ref can
consistently be used to get a checked reference to an entry for both
vectors and matrices

The
auto& backend_ref = backend();
in many methods has to be protected by the mutex, otherwise the reference
may not be valid when it is used. To avoid a double lock, the
backend() and ensure_uniqueness() methods now do not lock anymore and thus
need to be protected by a mutex if necessary