fixed mz calc with n_term mod

[picciama]

PR Checklist

• This comment contains a description of changes (with reason)
• Referenced issue is linked
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

• added TMT test for annotation and fixed adding the n-term mod mass to y2-yn ions

Technical details

Instead of just adding a potential n-term mod mass to the first aa position before adding all other mod masses to the respecitve aa masses, the n-term mod mass is removed from the mod dictionary mapping the mod positions in the aa sequence to their mass. Since n-term mods are stored at position -2, this lead to y2-yn ions getting it added by mistake after calculating the cumsum.

Additional context