Rename some things

• Rename effect_tox_observed_averaged and effect_tox_env_observed_averaged. Maybe "smoothed" instead of "averaged"?
• Rename all variables calles sys_stress_* because the "stress" is redundant and thus confusing.
• Attach "before_fit" to the names output$sys_stress_tox and output$sys_stress_tox_env. Also include a fitted version in output.
• In ec() rename target_effect to response_level making it similar to drc::ED().
• The names of the objects in the ecxsys output including the column names in output$curves are messy. Make them easier to understand and less similar to each other, if possible.
• I may want to be able to plot other stresses, not only sys. Therefore I should rename the function plot_system_stress to plot_stress.
• "simple" -> "LL5"
• effect_tox_simple -> effect_tox_LL5, effect_tox_mod_simple -> effect_tox_LL5_mod, and same with env.
• Updated documentation.
• Updated tests.