add mhchem and siunitx since I use them very often

tex/header/stdmath.sty

@@ -73,6 +73,8 @@
 \RequirePackage{mathtools}    % for := (defined as)
 \RequirePackage{amsthm}                   % for theorems environments
 \RequirePackage{enumerate}                  % for different kind of numbering in enumerations
+\RequirePackage{mhchem}  % chemical formulars. Provides \ce
+\RequirePackage[per-mode=fraction]{siunitx}  % provides nice typesetting for units and numbers
 
 % bibliography
 \newcommand{\bibfile}{.maindir/tex/bibliography.bib}