Do away with rxref pointgrid adjoint taking x_up as argument.
Do away with rxref pointgrid adjoint taking x_up as argument.

Make it use x as output argument, just like txref. There are too many
special cases to worry about for using x_up, and it's not worth the
hassle considering txref is preferred anyway.

ryanvolz/radarmodel,ryanvolz/radarmodel