fixed calculation of [M-H] ions when only one anionic modification used

Created by: margotbligh

fixed mistake in calculation of [M-H] ions when only one anionic modification is used. previously would only calculate [M-H] ion, and not [M-2H] etc. now works the same as for if multiple anionic modifications are ppresent i.e. will calculate [M-nH] ions where n is 1 to the maximum number of anionic groups on any oligo