I have created a test for a single neuron
Refactorized code.
Corrected some test. I wrote some wrong test methods like assert that a
new neuron has not an empty list.
Corrected the class.

Now the class pass all the test. It need to be refactored in some cases.

For example, the neuron always updated the output when a input change.
It can improve the perfomance if it update the value only when you want
to see it and it was changed it inputs.