ADIOS2 Updates for Code Coupling

[Steven-Roberts]

This expands some of the ADIOS2 functionality that was in adios_object.cxx and options_adios.cxx to have a simple public interface for reading and writing standard BOUT++ data types such as Field2D, Field3D, and Array. This is mainly intended for code coupling where the usage would look something like

// Write (via a stream) a Field3D to another code
Field3D temperature = ...;
bout::ADIOSStream &writerStream = bout::ADIOSStream::ADIOSGetStream("temperature", adios2::Mode::Write, "SST");
writerStream.beginStep();
writerStream.Put("temperature", temperature);
writerStream.endStep();

// Read (via a stream) a Field3D from another code
Field3D forcing = ...;
bout::ADIOSStream &readerStream = bout::ADIOSStream::ADIOSGetStream("forcing", adios2::Mode::Read, "SST");
readerStream.beginStep();
readerStream.Get("forcing", forcing);
readerStream.endStep();