Bug fix

• implementation of the add_hc flag in InteractionTerm involved a bug that could lead to incorrect results

Minor changes/additions

• update to plotting routines now supports various extra plotting options such as 'linestyles' etc.
• added TunableTransmon class for flux-tunable transmon, including junction asymmetry
• limit support to Python >= 3.6
• corrections to documentation of FullZeroPi
• added missing jupyter notebook illustrating use of HilbertSpace and ParameterSweep
• overhaul of file IO system now allows saving and loading various scqubit data via a custom h5 file format
• ipywidget support for creating qubits inside jupyter (try, for example, scqubits.Transmon.create())