radiation hyperbolic BCs need to know about flux BCs

When we a radiation flux entering through a boundary into a vacuum state, the boundary conditions on the radiation energy in the hyperbolic part need to know about this. There is some discussion of this in the BC section of Advances in Radiation Modeling in ALEGRA: A Final Report for LDRD-67120, Efficient Implicit Multigroup Radiation Calculations (https://ntrl.ntis.gov/NTRL/dashboard/searchResults/titleDetail/DE2006876350.xhtml)

from @WeiqunZhang

I think here is how it can be done.
Using (2.1.19) and (2.1.20), one can compute F given F_in or F_out and E_r.
F is the radiation flux across the boundary. So we can figure out grad E. Then we can use that to fill hyperbolic boundary.