Deduplication, support select_streams on single pass, support headers-only

[cboulay]

This PR deduplicates code, allows use of select_streams without double traversal of file (#60) and adds the ability to extract headers only.

It was a bit tricky to do while holding onto a couple top-level functions. I hope I did it to everyone's satisfaction, though it's maybe a little messier than it could have been otherwise.

I think the logger is probably a little too verbose by default, especially when skipping over non-selected streams or grabbing only headers. But I'd like some feedback before I clean that up.

I also think some of the top level functions (even the _ prefixed ones) can be re-ordered more logically, but I wanted to limit the already-large diff.

There are quite a few changes here so I'm tagging lots of reviewers. Please test on your data files. If we're happy with my previous PR then I'll merge that in here so we can test on data files with problematic clock synchronization.