Introduces epislon checking against *relative* masses in Material::extract

While utilizing different units has helped reduce some mass extraction issues, using a relative epsilon has proven vastly superior to using an absolute epsilon. With the two combined, very similar materials can be extracted from one another much more smoothly. This is related to @gonuke's suggestion the last time we talked about this issue in Madison. We didn't give him enough credit at the time, but he was totally right when he said I'd run into this problem once the units were no longer an issue. Thanks @gonuke :) .

For illustration of the necessity of this behavior, I share with you the following effect this has had on some results from Cyder (hint, the after one is more correct) : 

Before : 
![image](https://f.cloud.github.com/assets/393899/388312/99f284f6-a6d6-11e2-8348-b763a7cd9ab8.png)


After : 
![image](https://f.cloud.github.com/assets/393899/388228/9dfdfc30-a6d4-11e2-89e6-fcbdceb78943.png)