Simplify code for linear transformers

[mattansb]

All of the following are linear transformers:

• center()
• standardize()
• slide()
• reverse()
• rescale()
• normalize()

So hypothetically we should be able to simplify their code to a common underlying function. Idially, each function would compute the needed shift and scale values, and these will be passed to standardize(..., center = <shift>, scale = <scale>).
This will have 0 effect on usability, but will make maintenance of the code much easier.

Function	Supports weights?	Shift	Scale
center()	✅	mean(x) or median(x)	1
standardize()	✅	mean(x) or median(x)	sd(x) or mad(x)
slide()	❌	min(x) - lowest	1
reverse()	❌	-(max(x)+min(x))	-1
rescale()	❌	min(x)*<scale>-to[1]	(to[2]-to[1])/(max(x)-min(x))
normalize()	❌	same as rescale(x,to=c(0,1)+c(-1,1)*include_bounds) where include_bounds is a number [0,1] or 0.5/length(x)

In any place where a reference= arg is provided, replace x with reference above.

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All functions would have to deal with missing values, non-finite values, and labels.

[strengejacke]

I looked at the code, and I'm not sure if there's really much we can simplify. Due to the possible transformation inside formulas, we have the dw_transformer class, which requires different attributes (depending on the transformation function), so we need these information anyway, and can't "standardize" this code. Furthermore, there are a few exceptions that we must handle anyway, e.g. reversing for factors (where min()/max() don't work).

You can try to improve things here, but my first impression is that not much simplification is possible beyond what we already did.