This function gets mass of an undeuterated peptide based on its spectrum.
get_undeuterated_mass(theoretical_spectra)a data table or a list of data tables of
theoretical spectra created by the function
simulate_theoretical_spectra.
`data.table` of calculated mass value for the first peak (the smallest one) as it is usually the peak corresponding to the monoisotopic mass.
For the calculations the formula below is used
$$undeuterated_mass = charge * Mz - p_m$$
where Mz is mass-to-charge ratio for the peaks from the provided
theoretical spectrum and p_m is the mass of proton equal to 1.007276.