Correct a-B fragment masses for DNA

[poshul]

Description

Fixes an existing issue where custom deoxyribose mods (with the naming convention [d*]) were not producing the correct masses for base loses.

Checklist:

• Make sure that you are listed in the AUTHORS file
• Add relevant changes and new features to the CHANGELOG file
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes
• Updated or added python bindings for changed or new classes. (Tick if no updates were necessary.)

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[hroest]

could you add a regression test?

[poshul]

could you add a regression test?

Done, also added test for base methylation.

[timosachsenberg]

looks correct to me