* The `curves` data frame in the output of `ecxsys()` now contains a column with the concentrations which are used for the plot functions in this package. This is useful for generating a nicer concentration axis.
* Changes to `ec()`:
* Renamed `response_value` to `effect` in the output list.
*`response_level` of 0 and 100 are now allowed. 0 returns the concentration 0 and 100 returns the concentration `Inf`.
* It is now possible to set the reference to the maximum possible value, usually corresponding to the `effect_max` argument in `ecxsys()`
*`response_level` of 0 or 100 is now allowed. 0 returns the concentration 0 and 100 returns the concentration `Inf`. Previously this resulted in an error.
* It is now possible to set the reference to a custom value, for example 100.
