client.py:
1. minor stylistic changes
2. do not crash if pandas cannot be imported
3. bug fix to export_tsv
export_tsv raises NotImplementedError if necessary
client.py: documentatino for topN query
aggregators.py: re-name doublesum -> sum
filters.py: bug fix to show() method
postaggregator.py: Field and Const postaggregators call super constructor