Myokit

- Refactored `myokit.lib`.
- Added APD calculating function.
- Added APD measurement to simulation using CVode's root finding.
- Added padding function to `save_csv`.
- Created `SimulationLog` class.
- Various small bugfixes.

- Refactored import/export modules.
- Updated documentation.
- Added method to interpolate logged results.
- Improved performance in CVODE simulations.
- Added periodic logging option to simulation.
- Improved Explorer GUI.
- Added method to run euler/rk4 simulations.
- Added OrderedPiecewise class.
- Added progress bar to gui.
- Various small bugfixes.

- Bugfix in metadata export.

- Various improvements and bugfixes.

- Rewrote cellml import.
- Worked on methods to fit simplify functions.
- Updates to `mmt` syntax
- Added multi-line comments to `mmt` syntax.
- Various small bugfixes.

- Added tests.
- Added `piecewise()` expressions.
- Various improvements and bugfixes.