TST/REF: Deprivate check_pairwise_moment and remove ConsistencyBase
the `check_pairwise_moment` function will be the last function moving out of big `ConsistencyBase` class, and after this, `ConsistencyBase` class only has the `_create_data` function which creates some mock data, so we could entirely remove `ConsistencyBase` and use `Base` instead. 

cc @jreback 