Dose

1. Adding a function to Material that calculates dose per gram when given dose type (required, given as a string) and data source (optional) as input. 
2. Added Bq to Ci and Ci to Bq constants.

Feel free to suggest an alternative design. 
Both of these should follow the numpydoc convention.

```python
<name> : <type>, [optional]
    <description>
```

You should not put required, but rather put optional.  Also, ``{EPA=0, DOE=1, GENII=2}`` is not Python syntax, so this could be confusing.