Use output filename if provided

[IdoBar]

Un-commented the if() statement to check if an output filename was provided.
Currently a new unique date-based name is being created regardless of the user input.

[thierrygosselin]

Hi Ido, I had voluntarily commented the filename argument in write_snprelate, I've had issues with users providing filenames, I decided to have it decided by default by the function.

I'm going to fix this, however note that in my next release this will not be available. I'm removing SNPRelate dependencies from all of my packages.

I love the package and personally I've never had problem installing/using... apart from the problem on how the Fst is calculate, which is still unresolved on my end.

Lately, I came to the conclusion that It was way too much work for me trying to troubleshoot it's installation for users.

I will leave the code write_snprelate with a link to download on my GitHub page for interested users.

Cheers,
Thierry