Scqubits

**BUG FIXES**
* Fixed a bug that overwrote `<ParameterSweep>["evals"]` data with transition data when using plot_transitions().
* Fixed proper integration of `ParameterSweep` into `CentralDispatch`, enabling proper warnings to the user when internal computed sweep data is out of sync with associated quantum system parameters.
* Fixed a bug that could occur when a `ParameterSweep` was applied to a `HilbertSpace` object involving only a single subsystem.

**UNDER THE HOOD**
* Enable use of `weakref` in `CentralDispatch` for proper garbage collection.
* Extended pytests to basic `CentralDispatch` functionality

**ADDITIONS**
* New graphical user interface illustrating single-qubit functionality of scqubits.
* Introducing `NamedSlotsNdarray` as a convenient subclass of ndarray with name-based and value-based slicing, and immediate support for basic plots
* Added functionality for extracting dispersive energy parameters (such as Kerr coupling strengths)
* Improved support for transition plots (subsystem transitions, sidebands etc.)
* Added `Cos2PhiQubit` class.
* Added `KerrOscillator` class
* Added `GenericQubit` (two-level system) so that toy models such as the Jaynes-Cummings model can be readily realized with `HilbertSpace`.
* Added `n` and `phi` operators to the Oscillator class
* Added helper methods `convert_to_E_osc` and `convert_to_l_osc` for `Oscillator` initialization
* New and enhanced interface for defining interaction terms in HilbertSpace objects via .add_interaction()
* Added option to input interaction as a Qobj, or specify interaction terms as string expressions; also represented in HilbertSpace GUI



**IMPROVEMENTS**
* Convergence for `ZeroPi` is now faster, thanks to a correction to the expression for the grid spacing in discretization.py.
* Refactored `ParameterSweep` class, now allowing for multi-dimensional parameter sweeps
* Added a warning describing total=True being the default in t1 calculations


**BUG FIXES**
* Fix to type conversion error affecting the `number` operator in operators.py
* Rectified orientation of `matrix2d` plots to match axes labels
* Use of `mode` in values displayed in matrix element plots



**INTERNALS**
* New support for higher-order stencils in discretized derivatives.
* Improved formatting of `__str__` methods (called when "printing" an scqubits class instance).
* Under the hood: use of Python 3.6 compatible type annotations; unified formatting enabled by the `black` package
* Improvements to fileIO speeding up operations (increased memory cache) and requiring less disk space (avoid literal redundancies in stored data).

**Bug fixes**

- bug fix: `<qubit>.create()` failed in jupyter notebooks due to missing image files
- bug fix: corrected the form of the quasiparticle noise operator

**Major changes/additions**
- Coherence calculations for the majority of qubits. These allow for estimating coherence times and rates due to various noise channels.
- A new units system: users can specify energy units of their system Hamiltonian. These units are automatically considered when plotting and in coherence time calculations.
- Separated documentation and example jupyter notebooks into individual repositories, see scqubits-doc and scqubits-examples.

**Minor changes/additions**
- Introduced tests for real-valuedness of zero-pi Hamiltonians for speedup.
- New options in plotting (e.g. grid).

**Bug fixes**

- Fixed bug preventing the proper disabling of the progress bar.
- Various bug fixes and improvements of file IO operations.
- Fixed issue with color legend bar in .plot_matrixelements.

**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()`)