Add script to retrieve PDFs

scripts/get_pdfs.sh

-#!/bin/bash
+#!/bin/bin/env bash
 if [ ! -d "pdfs" ]; then
+    echo "Retrieving PDFs..."
     mkdir pdfs
     cd pdfs
     for i in {1..6}; do
         wget "http://pi1139.physik.uni-erlangen.de/data/latest/J${i}p.dat"
     done
-    cd ..
+else
+    echo "PDFs already downloaded."
 fi