Activation functions as static context instead of constant

We are currently using constant to declare those activation functions.
I think it will be much better if we use static context instead to declare them.

[Here](https://gist.github.com/xxMrPHDxx/f05b1ae9c722e4cbd99fd41f722fb5dc) I included the code in my gist as a reference.

The issue does not name files or tests. Start by locating the activation-function declarations in the JavaScript source and compare them with the linked gist; done means the declarations use the requested static context rather than constants without changing their behavior.

Scritto dal modello di indicizzazione a partire dal testo della issue.