deprecate Legolas.write_full_path and related behavior that uses it

[jrevels]

ref #26 (comment)

Actually, you know what - maybe we should just deprecate this function entirely. Why is Legolas in the business of creating intermediate paths for users at all? why not follow Base's lead and puts the onus on the caller to use mkpath beforehand if they need it?

Legolas' behavior is of course convenient when it's what you want, but maybe it really is generally more user-friendly (and at least more common) to error for local filesystem paths if they don't have the directories already made (and this shouldn't matter for S3Paths anyway)