Better predict_mixture documentation

description

• Add symmetry example to predict_mixture documentation.
• Add license notice to every code file and to the readme.

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

DESCRIPTION

@@ -2,7 +2,7 @@ Package: stressaddition
 Type: Package
 Title: Modeling Tri-Phasic Concentration-Response Relationships
 Version: 2.2.1
-Date: 2020-02-28
+Date: 2020-03-03
 Authors@R: c(person("Sebastian", 
                     "Henz", 
                     role = c("aut", "cre"),