Scqubits

**Bug fix**:
The `FullZeroPi` Hamiltonian contained an error affecting the case of nonzero `dC`.

**Additional changes and improvements**:
- improvement: Thanks to adjusted ARPACK options, diagonalization should be noticeably faster for `ZeroPi` and `FullZeroPi`.
- making `pathos` and `dill` the default for multiprocessing.

**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()`)

**Minor changes/additions**
- update to the setup script to properly include testing data with the PyPi release.

**Major changes/additions**
- scqubits now offers multiprocessing support for a number of methods.
- Introduced checks ensuring that umbrella objects like ``HilbertSpace`` and ``ParameterSweep`` instances do not accidentally go "out-of-sync" with respect to their basic components. When needed, warnings are thrown for the user to re-run sweeps or spectrum lookups.

**Under the hood:**
- Monitoring for changes of interdependent class instances is implemented through a central dispatch system. (disable: ``settings.DISPATCH_ENABLED``)
- Removed ``HilbertSpace`` reference from within `InteractionTerm` (throws deprecation warning if still used)
- Made ``HilbertSpace`` inherit from ``tuple`` rather than ``list``; composition changes to ``HilbertSpace`` warrant generating a new ``HilbertSpace`` instance
- Shifted ``InteractionTerm.hamiltonian`` to ``HilbertSpace.interaction_hamiltonian``
- Created ``DataStore`` as general purpose parent class to ``SpectrumData``
- No longer store custom data inside ``ParameterSweep``, ``sweep_generators.py`` functions return ``DataStore`` objects

- fixed a bug in display of FluxQubit wavefunction
- internal refactoring

- new class InteractionTerm works in tandem with HilbertSpace to ease setup of composite systems with pairwise interactions
- new ParameterSweep class efficiently generates spectral data for performing a scan of a HilbertSpace object over an external parameters
- new Explorer class introduces interactive plots (see docs and demo ipynb)
- cleaned up implementation of file IO operations