Implement dump(N) to return first N frames of data.

Closes #146

Also adds tests to check that `dump()` and `dump(N)` return data for the expect number of frames.
Oh, right. For the single-node format, it's a lot faster to just read the entire table, instead of doing a whole bunch of single-frame queries. (Although I guess we don't know this empirically.) So this class got its own implementation of `dump()`.