Support data being associated with other data

[clegaspi]

Find a way to associate a quantity with condition(s) for which it was calculated, or if it represents an upper or lower bound (see #214).

Related, but maybe not exactly the same, find a way to associate data pairs, like columns in a pandas dataframe. This way we can represent spectral data, or possibly the value of a property, like thermal conductivity, over a range of conditions, like temperature.