Auto-generated info for directory hierarchy in ADIOS output

Created by: pnorbert

Challenge: reconstruct the hierarchy at read time fast. OpenPMD spends 30 seconds to do that currently.

Idea: ADIOS could save the hierarchy as a string variable as a JSON/YAML document at write time, that either the application can use to generate the hierarchy or ADIOS could use it to generate a tree instead of the plain map of variables and attributes.

This could speed up the variable/variable and attribute/variable association functions at read time as well.

@ax3l @psychocoderHPC @franzpoeschel @BeyondEspresso