Inconsistent naming in `Measurement`: `species_data` field name vs. `MeasurementData` class name

[torogi94]

The Measurementobject contains a field species_data with its adder method add_to_species_data. However, the expected type of the field is the class MeasurementData. While working absolutely fine, the different naming ("species" vs. "measurement") of the data for the measurement is inconsistent and confusing.

Suggested solution:
Change naming of MeasurementData to SpeciesData, since there can be many measured species within a single Measurement object?

[JR-1991]

That sounds reasonable to me. However, I would prefer adding this to @enzymeml-specs so that all client libraries can adapt to it. Feel free to open a PR, and we can continue discussing it there.