Make BP3Serializer::SerializeMetadataInData() public so InSituMPIWriter can use it.
Make BP3Serializer::SerializeMetadataInData() public so InSituMPIWriter can use it.

JasonRuonanWang/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2,ornladios/ADIOS2,JasonRuonanWang/ADIOS2,JasonRuonanWang/ADIOS2,ornladios/ADIOS2