Currently, the user learns that `SharedPV::post()` requires the posted
value to be created from the same type instance as the value given to
`SharedPV::open()` only when they try to do it differently and are
presented with an error message. This is an attempt to document this
requirement.
Added "safety" of passing move-able reference
is an illusion since no use_count()==1 test
is done. Instead extra (shallow) copies
were made for each subscriber.
Instead. Pass const reference, redefine
MonitorControlOp::post() to transfer ownership,
and make only a single copy in SharedPV::post().
Jure Varlec