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

R/ec.R

+# Copyright (C) 2020  Helmholtz-Zentrum fuer Umweltforschung GmbH - UFZ
+# See file inst/COPYRIGHTS for details.
+#
+# This file is part of the R package stressaddition.
+#
+# stressaddition is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+
 #' Effect Concentrations
 #'
 #' Estimate the concentration to reach a certain effect or stress level relative