Remove/rename default childIndex variable

[Jesper Öqvist]

In the generated inherited equation defining methods there is always a childIndex variable generated even if an equation does not actually use the child index. The below code compiles fine despite childIndex not being declared in the getB parameter list:

eq A.getB().x() = childIndex;

[Jesper Öqvist]

I added a regression test for this (tests/inh/regression_01f), and could not reproduce the problem.