Add seek and tell to Python CuFile object

[jakirkham]

To make it easier to drop CuFile objects in where other Python file objects are accepted, it would be nice to have seek and tell methods. This would allow a user to go to a file offset and also check what file offset they are at.

[madsbk]

I think it could be a good idea, but it will require some smart chunking under the hood. This is because all of our reads and writes (incl. all of the cuFile API) takes ranges.

[jakirkham]

Could we simply store an off_t offset inside the CuFile object that we then simply do some integer math with to determine where to read and write from?

Or is there some existing behavior under-the-hood that would complicate this approach?

[madsbk]

Sorry, yes that would work. I was more thinking of using it together with #577 where stream read and write would not use GDS or parallel IO if we don't do something clever :)