Add 1931.Bethe translation; update export_split source dir

scripts/export_split.py

@@ -17,7 +17,7 @@ target="_blank">Creative Commons Attribution 4.0 International License</a>.
 """
 
 BASEDIR = "./"
-SRCDIR = f"{BASEDIR}src/org/"
+SRCDIR = f"{BASEDIR}src/"
 
 tree = parse(f"{SRCDIR}index.html")
 

src/index.org

@@ -7359,6 +7359,7 @@ Zeit. für Physik
 
 
 **Translations into English:**
+- by Vince Frederick (1992): ON THE THEORY OF METALS, I., in book: Daniel C. Mattis, /The Many-Body Problem - An Encyclopedia of Exactly Solved Models in One Dimension/, World Scientific (1993), ISBN 981-02-0975-4 (paperback: 981-02-1476-6).
 - by [[https://homepages.dias.ie/dorlas/][Teunis C. Dorlas]] (2009): [[https://homepages.dias.ie/dorlas/Papers/Bethe.pdf][On the Theory of Metals. I. Eigenvalues and eigenfunctions of a linear chain of atoms]]