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 returned reference may be invalid once the vector is changed, even
in single-threaded environments
- In the multi-threaded case, the reference may be invalid immediately
- As a replacement, add get_entry and set_entry to VectorAbstractions