Adios 1.7.0 support

Fix for #491

Add support for adios 1.7.0 data transforms and use MPI_AGGREGATE transport method by default.
Use fewer variables in ADIOS file (no vars for field sizes).
Is it actually possible to define a runtime value as a default or only compile time constants?
I think a useful default value could be the `MPI_Size` of `MPI_COMM_WORLD` if that is possible.