Add custom config file keyword argument

[gplssm]

Move oemof.db.connection() to close#13. Then ego.io and add filename keyword argument to it.

[Ludee]

Is this issue closed? @gplssm
If not I move it to v0.3

[nesnoj]

Related to #13 (v0.3)

[gplssm]

ego.io now () has it's own config file that is created when egoio.tools.db.connection() is called and no config file exists. Data processing now uses an OEDB connection via egoio.

Tested implementation by following query script

from egoio.tools.db import connection
from egoio.db_tables.scenario import EgoSlpParameter
from sqlalchemy.orm import sessionmaker
import pandas as pd

Session = sessionmaker(bind=connection(section='oep'))
session=Session()

data = session.query(EgoSlpParameter)

print(pd.read_sql_query(data.statement, session.bind))

[gplssm]

We won't resolve this entirely within v0.3.0 due to necessary changes in ego.powerflow. So, I postponed it.

[nesnoj]

related to #123