Introduce data attributes

We can't call the field "attributes", mongo uses that name. So I chose data_attributes.

They'll work in the same way as dimensions and concept schemes.
Data Attributes

Each dataset will have a number of data attributes, like provisional, seasonal adjustment etc.
Data attributes can be mapped to concept schemes

Concept schemes are optional, as the attribute notes, for example, needs no scheme.
Refactor: Rename structure to dimenions

We now have an data_attributes hash so having "structure" as a variable name seemed confusing.

Trying to be clear with what is what. Wat? No. Exactly.
Assigning of data attributes onto observation

Dataset's has_field? now checks both dimensions and data attributes allowing us to use method missing to assign those fields onto an observation.

Go Go Meta programming.
Merge pull request #1 from ONSdigital/dataset_attributes

Dataset attributes