Support Entity-Attribute-Value Tables

[totalhack]

Currently using entity-attribute-value tables (see example image) requires creating views for each attribute and putting those views in your warehouse config as individual dimension tables. It would be nice if zillion had a new "attribute" table type that could automatically adjust the warehouse definition based on the attributes that are supported.

To support this we'd need to:

1. Adjust core.py:TableTypes to support a new type
2. Adjust configs.py to support a new table type in the config
3. Review/adjust Datasource.find_neighbor_tables to make sure attribute tables can join as needed
4. (Hard part) Review/adjust the logic that builds tables and joins that can meet the required grain -- this will need special logic to look at attribute tables since they store their attributes as rows instead of columns.