Modernize HltDAQ

• prefer std::memcpy instead of std::copy_n to copy trivially copyable objects into byte buffer (and avoid the need for any casts) -- as std:memcpy is explicitly sanctioned to be used to (implicity) create (trivially copyable) objects in the buffer it writes into
• generalize process overload set so that processVector is no longer needed
• use boost::pfr to generically implement process for aggregates
• add as_tuple method to non-aggegrate packed data classes to allow for a single generic implementation of PackedDataChecksum::processObject