BaseMode.read_group sums numeric fields unconditionally

[Richard Lewis]

In the ORM, BaseModel.read_group, fields of type INTEGER or FLOAT have aggregations computed automatically using either the column's supplied grouping_operator or SUM. It would be better if this behaviour were optional, or at least could be disabled.

The attached patch adds an 'allow_aggreation' parameter to the _column class constructor which limits aggregation in read_group to only those columns having this property as True. For fields.integer and fields.float, is it set to True by default meaning that current behaviour will not change.