geoalchemy2 support

## Help! geoalchemy2 support!

```

class CoordinateMixin():
location = db.Column('location', Geography(geometry_type='POINT' ,srid=4326, spatial_index=True, dimension=2), doc='gps coordinate')

```
Exception: Don't know how to convert the SQLAlchemy field user.location ()

I'v already add the following code in schema file.
```

from geoalchemy2 import Geography
from graphene_sqlalchemy.converter import get_column_doc, convert_sqlalchemy_type
from app.graphql.extesions import CoordinateJSON

@convert_sqlalchemy_type.register(Geography)
def convert_column_to_coordinatejson(type, column, registry=None):
return graphene.Field(CoordinateJSON, description=get_column_doc(column))

schema = graphene.Schema(query=RootQuery, types=types, mutation=MyMutation)

```

Start with the schema file and the reported conversion exception, then inspect how graphene_sqlalchemy.converter dispatches SQLAlchemy column types. Reproduce the Geography example and verify that schema creation handles the location field and produces the intended CoordinateJSON field.

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