std::complex< T > as attributes

Created by: ax3l

Hi,

is there a specific reason why we do not allow std::complex< T > types as attributes in ADIOS2? Those are primitive types and according to my tests in openPMD-api, using complex types as attributes was possible in ADIOS1.

Can we potentially just update the macros that specialize the overloads to allow complex types for attributes as well?