Fixed order of current_shape assignment for _AggregationStreamsHandler.

Closes #84.


The changes themselves are trivial - the key is that I've removed the assignment of ``current_shape``, which was being assigned before finialising the previous chunk. I could just as easily have moved it to after the finialise call, but decided that it was only necessarily assigned for some of the handlers, so removed it as a standard ``AggregatedStreamsHandler`` attribute.