Do not generate set methods for NTA children

[Jesper Öqvist]

It should not be possible to set a value for an NTA child. For example:

syn nta Opt<B> A.getBOpt() = new Opt();

...

A a = new A();
a.setBOpt(new Opt<B>(new B()));

The setBOpt method in the above example should not be generated. Instead the developer can write:

a.setB(new B());

This is both safer and more concise.

[Jesper Öqvist]

Using the setters for NTA children is unsafe because NTAs are treated as regular attributes and can be flushed.

[Jesper Öqvist]

Do not generate NTA setters methods

NTA values should only be computed using the corresponding NTA equation.

fixes #200 (bitbucket)

→ <<cset d8e85953e635>>