boost serialization of pagmo::detail::prob_inner<UDP> manual export required

[codethinki]

Describe the bug
cpp The pagmo::detail::prob_inner<UDP> class is not automatically exported to boost by pagmo.
To Reproduce
If you instantiate a archipelago with:

pagmo::archipelago arch{
  ISLANDS,
  pagmo::algorithm{pagmo::sade{}},
  pagmo::problem{MyUDP{}},
  POPULATION_SIZE,
}

the serialization with:

std::ofstream file{"myfile.bin", std::ios::ate | std::ios::binary};
boost::archive::binary_oarchive archive{file};
archive << arch;

crashes with boost complaining about a base class thats not registered (no type information, hints whatsoever).

i also don't see any documentation for the serialization feature but thats something else.

fix

add

BOOST_CLASS_EXPORT(pagmo::detail::prob_inner<MyUDP>)

altho the library should probably take care of this

Expected behavior
Automatically register the class for boost serialization,

Screenshots
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Environment (please complete the following information):

• OS: Windows
• Installation method: vcpkg
• Version: 2.19.1

Additional context
Add any other context about the problem here.