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.
Yeah, possibly...

The thing is that I had this crazy long discussion with @MrShish and @idlefingers about this naming. After going back and forth, both IRL and on GH, we finally settled on that `value` appeared the best term for anything that represents a real-world cost for something.

Also, the specificity that you suggest rather adds to the confusion in my mind. If named `netOfDiscount`, you would all of a sudden start to wonder whether VAT is included or not...

Just `value` is fine, because that's like the end of the funnel. It's what everything boild down to. The monetary value of the cart.

Well well. Naming is hard :)