Custom munster manual

I added a tab with the link to the tutorial.
This will be useful so that students do not get lost.
Moreover, "munster" appears explicitly in the version name,
to clarify that this is not 2.1.1

**We should revert this commit before merging munster on v2.1**

VERSION

@@ -5,4 +5,4 @@
 # (this is same as gromacs)
 # Notice that "plumed --version" will return only 2.X
 # and "plumed --long-version" will return the full string
-2.1.1
+2.1.munster

user-doc/go-doxygen

@@ -38,6 +38,7 @@ awk -v version=$(plumed --no-mpi info --version) '{
     print "      <li><a href=\"_syntax.html\"><span>Getting&#160;started</span></a></li>"
     print "      <li><a href=\"tutorials.html\"><span>Tutorials</span></a></li>"
     print "      <li><a href=\"glossary.html\"><span>Index&#160;of&#160;Actions</span></a></li>"
+    print "      <li><a href=\"munster.html\"><span>Munster&#160;Tutorial</span></a></li>"
   } else print
 }' $file > $file.tmp
 mv $file.tmp $file