h_Pipe class does not support diffusion

[n7275]

diffusion rate between two pipe-connected volumes should be proportional to:

someGlobalConstant * (in_t + out_t )/2 *pipe_size * exp(-abs(in_p - out_p))

this would be in an

if(two_ways)){ //... }

block at the end of void h_Pipe::refresh(double dt)