Improper use of `set_const`

https://github.com/eschnett/openPMD-api/blob/13ea8b4ecca968a5355825d639b3e9182d14b484/src/binding/julia/Datatype.cpp#L15

In the above line, `jlcxx::Module::set_const` is used on an `std::vector`. This was always something that was incompatible with Julia precompilation, and since libcxxwrap-julia 0.11 gives a compile error. A function returning the vector should be used instead.