Random-access for variable-based encoding (i.e. use SetStepSelection for ADIOS2 steps)

[franzpoeschel]

TODO:

• MPI-aware preparsing
• Probably for a later PR: Preparsing also in READ_LINEAR (i.e. eliminate duplicate Iterations better than now)
• Edge case: What if a particle species completely vanishes after some iteration, making it appear as an empty, i.e. constant component -> need to distinguish this better in the frontend
• Better testing
• Documentation, examples
• Replace group-based encoding with variable-based encoding more thoroughly for ADIOS2 (maybe follow-up)
• Can we make variable-based encoding the default output for ADIOS2? (maybe follow-up for easier reversibility)
• Check if the engine supports this mode
• Check if the datasets have the same number of steps as there are snapshots
• hehe maybe pre-parse the entire thing in linear encoding and only then switch to random-access?