Warn about intercepts out of radius

[jgosmann]

Especially the Exponential distribution (if one forgets to specify high) makes it easy to accidentally generate intercepts outside of the ensemble radius. This leads to flipped response curves.

I think Nengo should at least warn or even raise an exception when this is happening.

[arvoelke]

This was also pointed out by #1212.

[jgosmann]

Closing this as a duplicate.