Extend UnlocalizedModification to represent complicated mods as set of simpler ones

[acesnik]

This would allow transitions from proteoforms identified with ET multimod sets that remove one PTM (e.g. dimethyl as {methyl, methyl} could have a transition -14 Da that removes a methyl).

[acesnik]

This is somewhat complicated:

• Right now, we can only remove Unlocalized PTMs with the same mass... Therefore, if have a -14 Da transition from a proteoform with a dimethyl annotation (+28 Da), it does not recognize that it should remove one of the existing methyl groups.
• Representing this is tough because you have to represent the relationship to other PTMs in the list and number of those PTMs. I think this could best be done by making a new tag in the PTM lists in mzLib that draws the connection to other PTMs in the list with a count of the number of them.