Hide 'write once, do not directly read' instance of RawEvent behind an opaque view

Address issue #179 (closed): make sure that a RawEvent which 'supports' RawBank::Views in the TES cannot be modified (which would invalidate the views) by writing it using writeViewFor with a view that does not allow access. Some syntactic sugar i.e. template <typename T> using writeOnly = writeViewFor<T,OpaqueView>; has been added to LHCbAlgs/Traits.h (which is included by the functional algorithms types defined in LHCbAlgs) for convenience.

Superseded by !3873 (merged).