Support import/export of peak annotation columns

[jorainer]

Support import/export of additional peak variables (in addition to mz and intensity).

@sneumann @michaelwitting do you know of any format conventions if we want to support e.g. formulas to be assigned to individual peaks? is this just an additional column in the peaks section?

[michaelwitting]

No aware of any conventions at the moment.
Is this also relevant to other backends, e.g. MsBackendMassbank. We can also have formulas for peaks there.

[sneumann]

The https://github.com/HUPO-PSI/mzPAF people are currently putting together an annotation proposal,
IIRC they also mentioned MSP output somewhere. peak annotation is certainly not standardised ...