style picking

[JanMarvin]

Provides wb_get_cell_style() and wb_set_cell_style().

wb <- wb_load(system.file("extdata", "oxlsx2_sheet.xlsx", package = "openxlsx2"))
styles <- wb_get_cell_style(wb, 1, "A10:G15")
wb <- wb_set_cell_style(wb, 1, "A30:G35", styles)
wb$open()

Needs

• wrapper test
• test
• vignette entry
• copy roxygen

[JanMarvin]

@jmbarbone fyi: I'm not entirely happy with the return of wb_get_cell_style(). It simply returns the style id as character. This makes sense in the way openxlsx2 handles styles, but (a) it breaks the flow, chaining is not possible this way, and (b) we might want to make this a class we can check for and provide a custom print model, so that the user wont be entirely confused when seeing the results and debugging chains or pipes.

We might want to look at this for possible improvements in the future, but for this pull request and most likely for the upcoming release I'll go with the implemented approach. It works and is a slight improvement over the current status quo. That's currently good enough for me.