Writing single value attributes to adios2 output
This adds functions for writing (almost) any single value attributes to adios2 output with the AdiosWriter under src/hyteg/dataexport/ADIOS2. Valid data types are defined in "adiosHelpers::adiostype_t". Having these attributes available could be handy for post-processing and analysis, as well as for checkpointing. I also added a non-exhaustive selection of attributes to be written to both the P1- and P2-output in the TerraNeo app.