upcxx::boost_serialization<T> adapter unimplemented

[Dan Bonachea]

We plan to eventually provide an adapter to enable UPC++ serialization of classes with pre-existing Boost serialization support.

This feature is not yet implemented in release 2020.3.0.

This issue documents the known compliance gap with the specification for this class.

[Dan Bonachea]

As of pull request #177, the compliance gap is documented in the ChangeLog and an empty header provides an explanatory #error.

The issue remains open for implementing the missing feature in the future.

[Dan Bonachea]

We've decided not to include the Boost serialization adapter in the main library at this time.

Extras PR 25 proposes a Boost serialization adapter, to be made available via upcxx-extras