Add extraction radius extraction for Q and S fluxes in BBSGrazingFluxes example

[tamaraevst]

Currently we calculate global integrals (over the whole volume) for the charge (Q) and source terms (S) in flux calculations, see e.g. this line. For future versions of the code, it would be good to have the integrals computed at different extraction radii, provided by the flux_extraction_radii parameter.

This capability would be nice for consistency with what is already done for the flux, F here, and for a better interpretation of the QFS system described in arXiv:2203.13845.