Myokit

- Added PySide2 support.
- Deprecated PyQt4 and PySide.
- Added a method `Model.remove_derivative_references()`.
- Bugfix to `Model.repr()` for models with no name.
- Added `Model.timex()`, `labelx()` and `bindingx()`, which work like `time()`, `label()` and `binding()` but raise an exception if no appropriate variable is found.
- Deprecated `lib.multi.time()`, `label()`, and `binding()`.

- Added method `Variable.convert_unit()` that changes a variable's units and updates the model equations accordingly.
- `Unit.conversion_factor` now returns a `Quantity` instead of a float, and accepts helper `Quantity` objects for incompatible conversions.
- Added `Unit.clarify()` method that gives clear representation.
- Added `Unit.multiplier_log_10()` method.
- Added `rtruediv` and `pow` operators to `Quantity` class.
- Small bugfixes to `myokit.lib.hh`.
- Stopped requiring HH alphas/betas and taus/infs to depend on V (allows drug-binding work).
- Bugfix: Time variable in CellML export no longer has equation or initial value.
- CellML export: components now ordered alphabetically.
- Variables with an `oxmeta: time` meta annotation are now exported to CellML with an oxmeta RDF annotation.
- CellML import now allows `deca` prefix.
- Added CellML identifier checks to cellml import.
- Renamed `DataLog.find()` to `find_after()`.
- Added DataLog.interpolate_at(name, time) method.
- Improved colormap used in `plots.cumulative_current()`.
- Bugfix to 'myokit step' for models without a name meta property.
- Updated sign error handling in `myokit.step()`.
- Added IDE shortcuts for unit checking.
- IDE now jumps to unit errors, if found.
- Improved exception display in IDE.
- Var_info now includes unit.
- Fixed bug in `Unit.__repr__` for small multipliers.
- Improved notation of units when complaining about them.

- Added option to register external formats.
- Added option to avoid certain prefixes when generating unique variable names.
- `Model.expressions_for()` now accepts more than 1 argument, and handles dependencies on derivatives correctly.
- Removed deprecated method `Model.solvable_subset()`.

- Added debug option to `myokit compiler` command.

- Bugfix: Removing variables now also removes their bindings and labels.
- Added unit tests.
- Improved performance in `lib.markov` analytical simulations.
- Updated the `myo` script to use the python3 executable.
- Fixed a bug in the default script used when creating or importing a model.
- Made GNOME/KDE icons install using sys.executable instead of a hardcoded python command.
- Fixed handling of string encoding in cellml import.

- Myokit is now tested on Python 3.7, but no longer on 3.4.
- Updated default OpenCL paths for windows.
- GUI fixes for matplotlib 3.1.0+.
- Added `set_constant()` method to markov simulations.
- Added `log_times` option to `lib.markov.AnalyticalSimulation`, and started pre-allocating arrays.
- Added option to cumulative current plot to normalise currents.