Some bugfixes and more flexible concentration lengths

description

• Fixed unintended behaviour in plot_effect() and plot_stress() where supplying an empty vector caused the four standard curves to show. Now setting which to an empty vector or NULL shows just the axes. The default value is NA.
• Renamed the mixture_effect column in the predict_mixture output data frame to effect.
• Remove the restriction that the concentration vectors in predict_mixture() must be the same length. The longer length must be a multiple of the shorter length because the shorter vector gets recycled to the longer length.
• Modify gitlab_ci to give verbose package installation output. And remove the check for the previous R version because somehow drc and plotrix are not available there.

system details

• RStudio Version: 1.2.5033
• R Version: 3.6.3
• OS Version: Windows 10 x64 (build 18363)

checklist

• This merge introduces new features.
  • new features are documented
  • new features have tests
• update documentation including examples (if necessary)
• update and polish NEWS.md
• update version in DESCRIPTION
• update date in DESCRIPTION
• no remaining TODO, FIXME, or debug prints anywhere in the source files
• devtools::document()
• devtools::check() without errors or warnings

R/plot_ecxsys.R

@@ -31,8 +31,9 @@
 #'   \code{model$curves} data frame. There is also \code{"effect_tox_observed"}
 #'   and \code{"effect_tox_env_observed"} for the observed effects and
 #'   \code{"sys_tox_observed"} and \code{"sys_tox_env_observed"} for the
-#'   observed Sys. The default \code{NULL} only plots the most important curves.
-#'   Use \code{which = "all"} to display all curves.
+#'   observed Sys. The default \code{NA} only plots the most important curves.
+#'   Use \code{which = "all"} to display all curves. An empty vector or
+#'   \code{NULL} creates just the axes.
 #'
 #' @param show_legend Should the plot include a legend? Defaults to \code{FALSE}
 #'   because it may cover some parts of the plot depending on the plot size and