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