allow adding two abundances/deltas (e.g. with + or mass_balance()) not only if all attributes match perfectly but also if attributes are a pair of "" and "something" and copy "something" into the new object (discard "")
- think about whether we really want to allow this!
allow adding two abundances/deltas (e.g. with + or mass_balance()) not only if all attributes match perfectly but also if attributes are a pair of "" and "something" and copy "something" into the new object (discard "")