Change signature of methods to use IO monad

[labra]

For methods that access to external resources, change their signature to use the IO monad.

[labra]

I started to refactor RDFReader in this branch by in the fs2 gitter channel several people have suggested to use just Stream[IO,A] instead of EitherT[IO, Throwable, Stream[IO,A]].

See this conversation for suggestions.