Remove early reflections from network before adding direct path response

[orchidas]

The network in its current form cannot model fade-in late reverberation which is observed when the source and receiver do not have a line of sight. This is because the early echoes from the FDN are discrete, and create the perception of a direct path, even when there is none. To fix this, the early response of the FDN should be removed. Instead of adding the direct path when calculating the network's transfer function, we should add it only when calculating the loss. Then, when we invert the network's transfer function back to the time domain, we can discard the direct path from the FDN, and replace it with the direct path from the measured RIR.