Unexpected immonium ion labels

Describe the bug
Neutral losses from immonium ions have unexpected ion labels: imm(A) is the label for the base ion; however, the neutral loss ions have the form [imm1-H2O][+]. These should be changed to include charge for the base ion and use the sequence residue in place of an index counter.

Expected behavior
The base immonium ions will have the form [imm(A)][+] and neutral losses [imm(A)-H2O][+].