Run pt2to3 on all .py files

Run as: `for f in $(find . -name '*.py'); do pt2to3 -i $f; done;`

It will be required to also run on some non .py files for
the qsub scripts in the refactor_simulations branch.
Update usage of tables in documentation
Update pytables requirement

PyTables 3 is now required.
Merge pull request #23 from HiSPARC/pytables3

Update code for PyTables 3