Fix bug in METIS path loss: broadcast 'num_walls'

Now 'num_walls' is broadcasted to have the same dimension as 'd' in the
_calc_PS7_path_loss_dB_same_floor method.

What that means is that if, for instance, 'd' has a shape
(12, 12, 15, 15) and num_walls has a shape of (12, 12, 1, 1) then
num_walls will be broadcasted to (12, 12, 15, 15)