Refactor file paths

https://github.com/miraep8/rendseq/blob/84c7ac1935d4e03b1daaab50c380bb867e2bba8e/rendseq/zscores.py#L253

As noted in #21 , file actions like this throughout this codebase are fragile (e.g. using slicing and such to figure out file paths).

I think it's very likely that there are better ways to make new directories, make a new file with a different extension, etc. After #21 merges things will work on Mac/Linux/Windows, but this should be refactored eventually.