Describe schema of tabular dataset

[lars-reimann]

Is your feature request related to a problem? Please describe.
Instead of just specifying something is a TabularDataset we want to treat it as a TabularDataset with some schema.

Describe the solution you'd like

• Option 1: Treat schema as a record type/class - attributes describe column names and their types. Then a tabular dataset can be modeled as a generic class TabularDataset<C sub Schema>.
• Option 2: Make TabularDataset a special type itself