change update_cell() logic. fixes #356

[JanMarvin]

Currently an idea.

A test is this:

library(openxlsx2)

mm <- matrix(1,10000, 100)

wb <- wb_workbook()$
  add_worksheet()$
  add_data(x = mm)

wb$add_data(x = mm)

• check if this is a valid replacement
• add tests
• remove dead code
• remove update_cell() examples
• creation and applying styles should only happen if there's no data on the sheet or if replace style is explicitly called

[JanMarvin]

Alright, I already love it. It removes a bunch of slow code and simplifies our data writing process from two different approaches to a single one.

[JanMarvin]

If nothing comes up, I'll merge this tomorrow. We can test and tweak this afterwards if we need to. The benefits of having it even though it might not yet be perfect outweigh potential issues.