Utility sub-program

[luizaandrade]

• Clean file paths form "" to "/"
• Check that folder exists and throw error
• Check that file exists and throw error
• Test file extension and throw error

[kbjarkefur]

Utility is a more commonly use name for this in computer science than auxiliary. And it should be one .do and not many .ado. So I renamed this issue to utility sub-programs

I have a gist with documentation on how to do this: https://gist.github.com/kbjarkefur/16b63c1fc89ab52c3d4cae9c74288452

[luizaandrade]

Thanks, Kris! That's super helpful.

[luizaandrade]

implementing in #201