`Designer` relative effect empiric design

[xandaau]

When we use the Designer class to design a parameter of interest, we operate on the effect input values in the following relative form: [1.01, 1.02, ...]. This is a pretty handy notation for a variable, but further we always use in our calculations and stat criteria not really these relative effects, but an absolute type of the effect.

As far as the theoretical approach is concerned, this may be fine, for empirical approaches we can make adjustments and start to distinguish between relative and absolute effects.

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For empirical methods, we can implement the same functionality in the Designer class as in Tester: handling "absolute" and "relative" effects.

One way to do this is to simply start instantiating a Tester inside empiric methods(mainly stat_criterion_power method) and pass all necessary arguments to it. The Tester class already has all the implemented functions for all the statistical tests in the package.

The notation of relative effects mentioned earlier could remain the same - but now it will be an additional possible effect_type argument that will be passed to the Designer and further to the Tester, which will have two possible values "absolute" (default) and "relative".