Discount coupon value

[pilou2]

I did more testings and found few small bugs. I went through all ot modules and three of them are modified.
Discount coupons had a lot to do. For any reason, the tax re-calculation modes were kind of reversed. I had some doubt at the beginning, but other modules code (GV and group discount) confirmed that.
Other discount modules, GV and Group Pricing needed few corrections too but lighter than coupons module.
I am ready to make new PRs (lost penny and coupon), it might be easier for others to review or modify. Although now that V2.0.0 is released, I am not sure anybody will still have some energy left...

[swguy]

Although now that V2.0.0 is released, I am not sure anybody will still have some energy left...

Please proceed! There's always another release around the corner. We appreciate the hard work you have put into fixing this problem.