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. 
This should probably be at the top of the module. Here you are renaming a datatype to be another, backwards compatible data type from Python 2.