Virtual Records (Variables)

[ax3l]

ADIOS2 v2.10.1+ supports virtual variables (e.g., to derive an energy from a momentum of particles or a Poynting vector form fields).
https://github.com/ornladios/ADIOS2/blame/master/testing/adios2/derived/TestBPDerivedCorrectness.cpp

ADIOS 2.10 only has vec, mag: vector of (x,y,z) and magnitude of vector.

We would like to expose this so users can define such derived (virtual) variables.

This will be a prerequisite for sophisticated queries.

cc @guj @franzpoeschel