closes #57
Use the names as default values.
NA will draw only the axes.
Update documentation.

According to CRAN rules all exported functions must specify a return value.

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 plotting control for the observed points and improve the generation of the legend. See the `which` argument in `?plot_ecxsys`.
* Renamed `sys_tox_not_fitted` and `sys_tox_env_not_fitted` to `sys_tox_observed` and `sys_tox_env_observed` in the output of `ecxsys()`.

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

Quick renaming of the non-exported function `adjust_smooth_concentrations` to `adjust_plot_concentrations`.

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.