Utility function to define mixes of standards

[jorainer]

Given: a list of standards (with their mass and/or chemical formula) Goal: define mixes (groups) of these standards ensuring they have non-overlapping m/z

Properties:

• allow definition of ions/adducts (in order to calculate the m/z for the standards).
• allow to define the number of standards per mix

Question is also if this should go into MetaboAnnotation or into MetaboCoreUtils. I have the feeling that maybe MetaboAnnotation might be better suited, because it's a higher level function and maybe special case, thus it would not fit well into MetaboCoreUtils. @michaelwitting @philouail any thoughts?

[michaelwitting]

We should allow the user to define the minimum difference in m/z