set batches to number of samples if smaller than default

[tyarkoni]

I ran into this while debugging a model, and it probably won't come up very often, but if the number of samples in a MultiTrace is < 100, and you try to call summary on it, the size of the batches argument isn't adjusted, which causes slicing to fail. Can presumably be fixed by setting batches to the min of batches and length of the trace. I'll submit a PR when I have a chance if no one else gets to it first (just opening this so I don't forget).

[twiecki]

Sounds like a valid fix.