Optimise implicit radiation

[themikelau]

• Build compacted neighbour list including "j-neighbours" (getj=.true.), so that atomic statements can be removed from calc_diffusion_term.
• Increase size allocated to neighbour arrays by a factor of 10 (icompact=1.2*10.*nneigh_average*npart) .
• Directly cache the mean of the kernel derivative 0.5*(dWidrlightrhorhom+dWjdrlightrhorhom) for calc_diffusion_term.
• Cache rhoj into varij2 instead of varij so that compute_flux only needs to access varij2