Custom dataset location impossible?

Nobody has claimed this yet.

### Steps to reproduce
```
import qcodes as qc
from qcodes.dataset.sqlite_base import init_db, connect

dataDB = 'C:/Users/Junkie/Documents/Programming/Python/QCoDeS/datapool/experiments.db';
qc.config["core"]["db_location"] = dataDB;

conn = connect(dataDB, debug=True)
init_db(conn)

from qcodes.dataset.experiment_container import Experiment
from qcodes.dataset.measurements import Measurement
from qcodes import new_data_set, new_experiment
```

### Expected behaviour
Use the database under the path dataDB for the storage of measurements..

### Actual behaviour
It uses the database created under the current working directory instead.

### System
**operating system**

**qcodes branch**
feature/dataset

**qcodes commit**

Start with qcodes.dataset.sqlite_base.init_db and connect, then trace how new_data_set and new_experiment select the database after qc.config["core"]["db_location"] is set. Reproduce the example using the supplied dataDB path and verify that measurements are stored there rather than in the current working directory.

Written by the indexing model from the issue text.