Changed functionality of getGrossValue and added getNetValue. Added and ...

...fixed documentation

@adamvoncorswant @mikaellothman 

Hope we didn't do duplicate work mikael, sorry if we did!

I think we need to call it net and not just value because there are a lot of functions named somethingValue.
Sorry, but isn't this naming a bit silly? I mean, providing the type of the return in the method name?