Comet parameter: precursor_charge
         
         - This parameter specifies the precursor charge range to search.
         
- This parameter expects to integer values as input.
         
- If the first input value is 0 then this parameter is ignored and all charge
         states are searched
         
- Only in the case where a spectrum does not have a precursor charge will all charges
         in the specified charge range be searched.
         
- If the first input value is not 0 then all charge states between (and inclusive of)
         the first and second input values are searched.  Again, only for those spectra with no
         specified precursor charge state.
         
- If a precursor charge is present for a particular spectrum, this parameter will
         not override that charge state and that spectrum will always be searched.
         
- With the default "0 0" values and a spectrum with no precursor charge, Comet will
         either search the spectrum as a 1+ or a 2+/3+.
         
- The default value is "0 0" if this parameter is missing.
         
Example:
         
precursor_charge = 0 0     search all charge ranges
         
precursor_charge = 0 2     search all charge ranges (because first entry is 0)
         
precursor_charge = 2 6     search 2+ to 6+ precursors
         
precursor_charge = 3 3     search 3+ precursors