Reduce redundancy in compound table imported from MoNa

[jorainer]

Data import from MoNa is available (issue #30), but the compound table contains a high degree of redundancy, i.e. it has one row for each MS2 spectrum.
We need to find a way to reduce this data by identifying unique compounds in that table. That has been elusive so far, because e.g. not all entries have an InChI key. Even those with an inchi key can differ in their exact mass (because of different precision of the numerical representation).