Result differences with respect to Lens about spinnaker_pdp2

[lplana]

@joannavioletmoy reports:

The output of one unit is 'flipped' in PDP2 relative to LENS in epoch 7 (nearly 1 in PDP2, nearly 0 in LENS). However, this appears to be because the dot products from which the value is calculated are also wildly divergent and have opposite signs (9.06 in PDP2 and -12.75 in LENS). This seems to reflect the fact that by this epoch, most of the weights in LENS are negative, whilst still being positive in PDP2. Closer inspection shows increasing differences in the weights with each epoch.

I have noticed big differences between the calculated values of the output derivatives in PDP2 and LENS. I've traced these right back to the first epoch and found that although at this stage the differences are relatively small, they seem to be much larger for units whose target is 1 than those who target 0. This appears to be the case across all the examples in the example set. Thus, I suspect the pattern may be being caused by the fact that we currently represent a target of 1 as 0.999969. However, I'm not 100% sure about this, nor have I established for definite whether this is the cause of the 'flipped' output value I am seeing in epoch 7, so I'm still looking into it.