NeuronSharedConstants doesn't need a constructor

[jbphet]

We generally us a pattern where the constants are just a static object, since there is no need to instantiate. See AreaBuilderSharedConstants.js for an example.

Found during code review, see #29.

[jbphet]

In continuing with the review, it appears that there is a NeuronSharedConstants and a NeuronConstants type, and the latter adheres to the requested pattern. These should probably be consolidated.

[AshrafSharf]

NeuronSharedCOnstants and NeuronConstants are consolidated into one class NeuronConstants which adheres to Object.freeze pattern

[jbphet]

Here is the commit for this one: 5363cb0.

[jbphet]

Looks good, closing.