`features` vs `fields`

[wilko77]

Currently we have a mixed terminology to refer to features / fields. The schema uses features, whereas the field_formats.py file mostly uses fields. It would be nice to unify this.

There is a lot of different terminology out there, depending on you allegiance.

CSV: talks about records which are divided by fields. Relational algebra: Terms like Domain, Table, Attribute, Key and Tuple are used SQL: row and column DBs: tuple or record and attribute or field ML: examples and features