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Update readme

Closes #47

See merge request !32

Release candidate

See merge request !31

version 3.0.0

Closes #24, #27, #36, #37, #38, #39, #40, #41, #43, and #45

See merge request !30

closes #38

v2.7.0 - return stresses of predict_mixture()

See merge request !29

Improve ec() and axis concentration handling

Closes #35

See merge request !28

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.

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.