extend testing, add new function

[ofenloch]

use std::filesystem to delete files before creating them

extend XLFormula tests

add function converting a string in a cell to a proper formula

[SimantoR]

It might be beneficial to use a flag to use filesystem as it's a relatively new API. TDM-GCC doesn't implement filesystem for C++17 if I remember correctly.

Or this might be an option: https://github.com/gulrak/filesystem

This should allow a consistent implementation across Windows, MacOS and Debian

[ofenloch]

@SimantoR

It might be beneficial to use a flag to use filesystem as it's a relatively new API. TDM-GCC doesn't implement filesystem for C++17 if I remember correctly.

Or this might be an option: https://github.com/gulrak/filesystem

This should allow a consistent implementation across Windows, MacOS and Debian

The CMakeLists.txt specifies C++17. Even if there are compilers not implementing the standard completely I don't think it's a good idea to deviate from the standard - although it might be benificial.