Attribute preparsing for variable-encoded files in ReadRandomAccess mode: Support modifiable attributes

[franzpoeschel]

This tries removing the major of two WIP restrictions for random-access reading of variable-encoded Series by preparsing.

TODO

• Docs
• Testing
• Forward engine parameters to the preparsing engine as well

Discussion: The ADIOS2 backend now contains two implementations for preparsing:

• The old implementation: Only /data/snapshot is preparsed, metadata is otherwise considered to be fully static. Advantage: The file needs to be opened in adios2::Mode::Read for preparsing only on a single rank, only the collected values for /data/snapshot are distributed in a single MPI operation at the end.
• The new implementation: Full support for reading modifiable attributes. Shows the correct values for attributes such as /data/time and has (restricted) support for groups that are present only in some steps (as long as they only consist of attributes and not arrays, i.e. as long as they are constant components). Disadvantage: Requires collectively reading the entire dataset and keeping the preparsed attributes for all steps in memory.

Hence:

• Should we keep the old implementation around for its performance benefits when the capabilities of the new implementation are not needed? – No, the old implementation is removed.