Function to get the absolute steps of a variable that is not written every step

[pnorbert]

ADIOS reading works with relative steps, i.e. if a file has 100 output steps, and variable X has 20 steps in it, then one addresses the steps from 0..19. What is missing, the opportunity for a reader to identify which actual steps X was written to.

The absolute step is not required for reading the data. It is desirable for applications/middleware to track back their stepping of that variable.

Asking for a public function for a variable, which returns a vector of the absolute steps the variable was written.

For example, openPMD needs to identify the absolute steps and currently does that by encoding the step in the name of the variable. A proper streaming solution with one-variable-many-steps requires this additional functionality (for openPMD to support its reading model).

@ax3l @franzpoeschel

[pnorbert]

fixed by #2508