Added ability to specify iterations for sb3

[ryash072007]

@edbeeching I think this would be useful when using sb3, instead of manually having to change the iteration value, it can be a command line argument which would make it easier to change it.

[edbeeching]

Thanks, I have been thinking more about how best to structure the library, would it be better to have one file per backend, so we can specify backend specific args more easily? What do you think?

[ryash072007]

Thanks, I have been thinking more about how best to structure the library, would it be better to have one file per backend, so we can specify backend specific args more easily? What do you think?

I was thinking about the same for sb3. It would make it easier in my opinion. I definitely support this method more then the current.

[edbeeching]

@ryash072007 this is superceded by #135, I will close. Feel free to test out #135 to be sure it has all the features you would like.