Added elastic constant example at finite temperature

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@14099 f3b2605a-c512-4ea7-a41b-209d697bcdaa

examples/ELASTIC_T/in.elastic

 # Compute elastic constant tensor for a crystal at finite temperature
 #
+# WARNING: This script attempts to measure small differences in 
+# statistical averages generated by finite temperature MD simulations. 
+# This creates many more opportunities for errors to occur, compared 
+# to the zero temperature version provided in examples/ELASTIC.
+# Hence, it is always a good idea to start with the zero temperature
+# calculation, before attempting to use this script.
+#
 # Written by Aidan Thompson (Sandia, athomps@sandia.gov)
 #
 #  This script uses the following three include files.