[SCons] Warn if untested SUNDIALS is used

We expect that minor version number release will not introduce breaking
changes, but the user should be warned that their version of SUNDIALS is
untested.

SConstruct

@@ -1112,6 +1112,9 @@ if env['system_sundials'] == 'y':
     if sundials_ver < LooseVersion('2.4') or sundials_ver >= LooseVersion('6.0'):
         print("""ERROR: Sundials version %r is not supported.""" % env['sundials_version'])
         sys.exit(1)
+    elif sundials_ver > LooseVersion('5.1'):
+        print("WARNING: Sundials version %r has not been tested." % env['sundials_version'])
+
     print("""INFO: Using system installation of Sundials version %s.""" % sundials_version)
 
     #Determine whether or not Sundials was built with BLAS/LAPACK