Gaussian activation function now plug-able

Gaussian function working.

A NNet and its associated FeedNet may now have different Num types.  The derivative of the Gaussian activation function requires the pre-activation input, which I'm storing as auxillary data in a type castable to floating point.

Casts have been added at the site of calls involving the FeedNet's Num type

Discredited output of test_sin does not look as good as before, but can't find a reason for the regression.
Continuous output of test_sin now availible (using Gaussian activation), however the fit is poor.
test_xor working well.