Some bugfixes and more flexible concentration lengths

See merge request !27

Control whether observed points are plotted

See merge request !26

* 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()`.

Improve predict_mixture

See merge request !25

* The concentration vectors are checked for NAs, which raise an error when found.
* The use of `drc::ED()` was replaced with a custom inverse weibull function, which does the same but better.
* `predict_mixture()` accepts multiple values for the concentration of the second toxicant. Both concentration vectors must be the same length.
* `predict_mixture()` returns a data frame with the concentrations and effects. Previously it was only a vector of effects.
* `predict_mixture()` received a new argument "effect_max" which scales the returned effect values.
* Renamed the arguments of `predict_mixture()` to use underscore letters a and b instad of 1 and 2. For example model_1 is now model_a.

Better predict_mixture documentation

See merge request !24

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

Change the way stress_tox_ca is calculated and check p and q of both models.

See merge request !23

Change the way stress_tox_ca is calculated to make the steps more logical. Does not change the results.

rename internal adjust_plot_concentrations

See merge request !22

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

curves_concentration_max and ec error

Closes #25 and #29

See merge request !21

* `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.

Fix regression in plot_effect()

See merge request !20

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

Closes #7, #21, and #23

See merge request !19

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

Add .gitlab-ci.yml

Closes #1

See merge request !18

update merge request template and readme

See merge request !15

Major rework of the source code and documentation. Refactor everything to improve usability and to make debugging and extending the package easier.

See merge request !3

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.

Last changes for first public version. Closes #11.

See merge request !12

Bugfix/warnings

Closes #15

See merge request !11

Fix failing test under devtools::check()

I ran the `testthat` tests but forgot to run `devtools::check()`. Somehow the options are handled differently in both cases.

See merge request !7