Cube.xsd cleanup

1. Add comments
2. Remove weight in cube and dimension
3. Is both derived and parent needed?
4. Remove cube_table from measure, expr_column
5. Remove cost from measure, dim_attribute
6. Should the type of measure be a list of allowable types instead of string
7. x_dim_attribute --> do we need table_ref, wont chain_ref alone suffice?
8. x_dim_attributes, dim_attr_names --> min_occurs should be 1
9. dimension_table, change dim_table to dimension
10. enumeration for update_periods
11. x_storage_elements description change
12. x_parition, change cube_table_name
13. Please check if x_partition should expose more parameters

Detailed description on update_period_elements and sample usage