add option to musrfit that the user can choose from the command line the number of threads to be used for OpenMP. Updated the docu accordingly.

doc/html/tutorial.html

@@ -14,7 +14,7 @@
     <script type="text/javascript">
       var DOCUMENTATION_OPTIONS = {
         URL_ROOT:    './',
-        VERSION:     '1.7.1',
+        VERSION:     '1.7.4',
         COLLAPSE_INDEX: false,
         FILE_SUFFIX: '.html',
         HAS_SOURCE:  true
@@ -448,8 +448,8 @@ For a complete description please refer to the manuals of <a class="reference in
     </div>
     <div class="footer">
         &copy; Copyright 2021, Andreas Suter.
-      Last updated on Apr 09, 2021.
+      Last updated on Aug 06, 2021.
       Created using <a href="http://sphinx-doc.org/">Sphinx</a> 1.2.3.
     </div>
   </body>
-</html>
+</html>