Better support for radioactivity units

[hunterpayne]

There are several missing types for doing calculations around radioactivity.

1. A unit measuring disintegration rates (ie Curie, Becquerel, and Rutherford) which is often called Activity
2. A unit measuring area * time (ie SquareMeterSeconds) which I call AreaTime (used to quantify exposure)
3. A unit measuring ParticleFlux (ie BecquerelsPerSquareMeterSecond)
4. A unit measuring equivalent/effective/committed dose

Also, adding ErgsPerGram to SpecificEnergy

Also, conversions between AreaTime, Area and Time

[hunterpayne]

#337

[garyKeorkunian]

Fixed by #337