Unverified input and cache graph data.

[wlngai]

The current Graphalytics assumes that the input graph and (cache) graph are correct, which is fine as if it is not then the corresponding benchmark runs will not pass validation. However, it is unclear to users why the validation failed, as they assume the input graph and (cache) graph are correct. These datasets can be accidentally corrupted for example when the caching process was interrupted.

A check-sum (e.g. sha1) should be implemented on these files for full validation.