compatibility with io table backend

[mfastudillo]

Hi

I was testing bw_aggregation with a database that uses the IOTable backend and I am finding some problems.

1. the functions of bwanalyzer seems to always use the aggregated version
2. the iterators that go over the exchanges seem also to use always the aggregated version.

I couldn't find an existing example of the IOTable backend so I extended the example here

https://github.com/mfastudillo/brightway2-io/blob/io/example.ipynb