miFit: McCode: test and add fmax

[farhi]

A few stuff to finalize the McCode config menu:

• test the model evaluation (changes in the structdlg).
• Make sure it displays the 'nan' evaluation when no opened data set.

Currently, we have a message after changing a parameter value::

??? Undefined function or method 'fieldnames' for input arguments of type 'double'.

Error in ==> iFunc.subsasgn at 99
      if isa(b, 'iFunc'), f=fields; else f=fieldnames(b); end

Error in ==> iFunc.set at 94
    this = subsasgn(this, s, prop_values{index});

Error in ==> mifit_Models_View_Parameters_Edit at 152
  set(handle, 'UserData',model);

??? Error while evaluating uitable CellEditCallback

[farhi]

nearly fixed with https://github.com/McStasMcXtrace/iFit/commit/d055b20cf180f8d915707e890a096d86932cd945