Unify algorithm for calculating average tax rate for products in a cart

[eternoendless]

Questions	Answers
Branch?	develop
Description?	There are currently two methods for calculating the average tax rate in a cart, Cart::getTaxesAverageUsed() and $cart->getAverageProductsTaxRate(). They used a different algorithm to calculate it, but it more or less ended up in "sum of all products without tax / sum of all taxes applied". This PR unifies the behavior and depreciates the static method.
Type?	bug fix
Category?	CO
BC breaks?	no
Deprecations?	no
Fixed ticket?	n/a
How to test?	Add many products to a cart, ideally with different tax rates, and with some reductions. In the order detail (FO), the reduction amount should be correct (or at least no different from before )

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This change is 

[marionf]

@eternoendless

Product price tax excl: 10€
VAT 20%
Product price tax incl: 12€
Cart rule: -2,53€ tax excl (= -3,04 tax incl)

Before this PR, it's ok:

With this PR, it's not ok:

[eternoendless]

Fixed!

It turns out that getTaxesAverageUsed() returned a tax rate (eg '20' for 20%) while the other method returns a tax multiplier (eg. 0.2 for 20%).

[Quetzacoalt91]

Thank you @eternoendless