Add validator for county congressional results
Add validator for county congressional results

Add a validator, ``validate_2000_primary_congress_county_results``,
that checks that county-level results for congressional offices in the
2000-03-07 Maryland primary have been created.

This validates the ``create_2000_primary_congress_county_results``
transform.

Addresses https://github.com/openelections/core/issues/36

cathydeng/openelections-core,datamade/openelections-core,openelections/openelections-core,openelections/openelections-core,datamade/openelections-core,cathydeng/openelections-core