Input Needed: How to report optimizable parameters in opt.xml or vp.h5 files?

**Is your feature request related to a problem? Please describe.**
For orbital rotations we need to report the total rotation at each step. I think this can be done entirely at the RotatedSPOs level, but what I don't understand is how wavefunction parameters are reported in the opt.xml and, more recently, the "vp.h5" files. I see methods like "checkInVariables", "checkOutVariables", and "resetParameters". These seem to be called in QMCCostFunctionBase. And the manual states that these functions handle variational parameter stuff. But looking around in the code, I can't seem to figure out where or how the parameters are reported at each stage of the optimization.

- Is opt.xml preferred over vp.h5? There seems to be both at the moment.
- In any case, how do I get the parameters to either of those files?

@markdewing @ye-luo Please help!