Comet parameter: add_Cterm_peptide
         
         - Specify a static modification to the C-terminus of all peptides.
         
- The specified mass is added to the unmodified C-terminal mass (mass of OH or 17.00274).
         
- The default value is "0.0" if this parameter is missing.
         
Example:
         
add_Cterm_peptide = 14.01