refactor check logic in SeriesSchemaBase.validate method

[cosmicBboy]

#189 revealed a bug in the series check logic that doesn't account for duplicated keys in the check object index. This seems like functionality to factor out into a pure function (static method would also do) so that it can be more easily tested for properties:

• the number of rows/elements in _check_obj should be less than or equal to the number of elements in the series object.