Also changed constructors of PayloadEntryProxy to protected.

Create doxygen documentation thus:
$ cd docs
$ doxygen ipaacaCpp.Doxyfile

Address mode must be selected explicitly using as_map/as_list, like so:
  for (auto value: iu->payload()["listItem"].as_list()) { .... }
  for (auto kv_pair: iu->payload()["mapItem"].as_map()) { .... }
Generic size() function (reports 0 for non-structured types)

PayloadTypeConversionError thrown if (trimmed) string parse is
incomplete; comparison operators == and != catch this error though.

(This was sufficient for basic interoperation, e.g. DM<->realizer)

Setting default channel name and default payload type works OK

Sniffer: only display quotes for string-type payload values
Injector: using unicode() and catching errors

TODO: compatibility with STR legacy payloads; more varied
access to inner payload objects (e.g. push_back)

(for remotely triggered local-IU handlers, in order to let the
RPC call of the remote side succeed as it should).

It is a fast header-only 3rd party lib which does not have
packages on all systems - so I just copied it into include/

It is Herwin's resend fix. Test sources are present but not
enabled during library build (will have to implement test build asap).