Add checks that outputs are writable & inputs exist if outputting to or reading from a directory

[tmaklin]

Hi,

Thanks for creating fastspar, very useful and easy to use!

I was having some trouble with writing/reading from files that are in a directory different from where fastspar was run because the directory paths either didn't exist or had typos in them. I added a bunch of simple checks to deal with this. These checks do the following:

1. fastspar will now check if the output for --correlation or --covariance is in a directory (path contains a '/') and exit with an error message if the directory (path stripped of all characters after the last '/') doesn't exist or isn't accessible. Previously the program ran fine without errors but didn't produce any output.
2. fastspar_bootstrap will check if --prefix is in a directory and exit if that directory doesn't exist. Previously the program produced a segmentation fault if this was the case.
3. fastspar_pvalues checks performs the same check as 1. for --outfile, and the same check as 2. for --prefix. Previously errors in --outfile caused the program to run fine but produce output, and errors in --prefix resulted in a slightly misleading error message about not having the correct number files in the folder.

The checks for the directory use <dirent.h> which is not available on windows systems, so I wrapped the relevant code in a preprocessor directive which removes it when compiling on windows.

[scwatts]

Thanks for spending the time putting this together. What do you think about removing the Win32 preprocessor guard and dropping any implicit/supposed Windows support entirely?

[tmaklin]

Hi, think I originally added it just to be sure that it doesn't break anything, I have no strong feelings about supporting or not supporting windows here 😁

C++17 has a more portable way to do these checks under the <filesystem> header so that would be preferable for making the checks work on Windows.

[scwatts]

Let's avoid attempts to preserve any potential/supposed support for Windows since that was never an explicit goal - there is value in supporting this but those changes would fit better in a more comprehensive PR. I'm not aware of any native Windows users, and regardless there is the option of WSL2 to run FastSpar.