moves table select to intermediate base implementation of Table

The new `SelectableTable` class implements the `select()` as shared function for `AttributeTable` and `FlatTable`. 

`BaseTable` is a relict when `DatabaseTable` existed and might be assimilated by `Flattable` as this is the main table implementation (`AttributeTable` acts more as a view on `FeatureCoverage` rather than to be used as independent table).