Control whether observed points are plotted

description

• 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().

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