Rename `setLabels` family functions to meet Tidyverse requirements

[GegznaV]

The family of setLabels functions should be renamed as it doesn't follow Tidyverse conventions, e.g., use only lower-case letters.

hySpc.dplyr/R/setLabels.R

Line 19 in ffbb179

setLabels <- function(.data, ...) {

I suggest renaming:

1. setLabels → labels_set() (I think, that labels_set() and labels_get() should live in the main hyperSpec package as they give core functionality to work with labels).
2. setLabels_transmute → labels_transmute() (Why is this function needed at all?)
3. setLabels_select → labels_select
4. setLabels_rename → labels_rename

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Yet, it is still a bit vague for me, what these functions do and why are they needed. The file with these functions lacks:

1. description/better explanation, why they are needed;
2. examples (at least one per function;
3. unit tests (at least one per function).