closes #38

Replace the "use_for_plotting" column in mod$curves with the plot axis concentrations, which is far more useful.
Add "refence" argument to ec() and some minor improvements.

Add symmetry example to predict_mixture documentation and add license notice to every code file and to the readme.

* `ecxsys()` gains a new argument `curves_concentration_max` which sets the maximum concentration for the curves.
* `ec()` raises an error if the requested curve is too short to calculate the result.
* Improve merge request template.
* Update readme example.

Expand the capabilities of the plot functions, add `predict_mixture()`, and fix documentation of ecxsys() and predict_ecxsys().

Major rework of the source code and documentation. Refactor everything to improve usability and to make debugging and extending the package easier. See NEWS.md for details.