allow model parameters in extra SLHA output blocks
add Au, Ad, Ae to list of extra SLHA output blocks

I also changed the syntax for output elements which do not have an
index:

Output an expression without an SLHA index:

   ExtraSLHAOutputBlocks = {
      {ALPHA, {{expression}}}
   };

Output an expression with one SLHA index:

   ExtraSLHAOutputBlocks = {
      {HMIX, {{1, expression}}}
   };

Output expressions with two SLHA indices:

   ExtraSLHAOutputBlocks = {
      {AU, {{1, 2, expression}}},
      {AD, {{1, 2, expression}}},
      {AE, {{1, 2, expression}}}
   };