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.
I must say that I still like just `value` better.
grossValue vs value is much clearer to me than grossValue vs netValue. In the latter case you won't know whether it's discounts or VAT that's taken into consideration, where I believe that that's unambiguous in the former.

I won't fight for it though :)