Add lopsided_kodis subroutine in lopsidediff.f90 that combines upwind
advection (lopsided) and Kreiss-Oliger dissipation (kodis) into one
function sharing a single fh buffer from symmetry_bd. This eliminates
27 redundant full-grid copies per RHS evaluation (108 per timestep).
For gxx/gyy/gzz variables: kodis stencil coefficients sum to zero
(1-6+15-20+15-6+1=0), so using gxx(=dxx+1) instead of dxx for the
dissipation buffer is mathematically exact.
Update bssn_rhs.f90 to use the merged lopsided_kodis calls.
Co-authored-by: ianchb <45872450+ianchb@users.noreply.github.com>
