Make NoOpOutputFormat Not Delete Output Directory

[spmallette]

Currently the NoOpOutputFormat deletes the output directory. "NoOp" seems to imply to me that "no operation" is performed so I was surprised the first time I used the "NoOp" and lost a sequence file i was still interested in using. (The second time it happened i was annoyed with myself for forgetting that behavior and pain from the first time.)

Anyway, consider making it so that NoOpOutputFormat performs in a true "NoOp" manner where no action is taken on the graph or output directory. At the moment, it seems more logical that it work that way.

[dkuppitz]

There are two main use cases for NoOp. One doesn't need any output at all, that's when you do global graph mutation. But the second use case is, when you only need the output of side effects (global graph analytics). So "a true NoOp manner" will not really work. We'll either need another OutputFormat (SideEffectOutputFormat?) or store side effect outputs in another directory.