Adding ODEs for helium bubbles

Using radioactive decay as a source term, these ODEs attempt to model the evolution of a helium bubble structure.

More to do - initial checkin
Checking in example for helium bubbles and making small changes to source

Correcting typos in source. Beginning to modify the Mechanics problem for helium bubbles.

The example will begin with a single element and move to 1-D diffusion along a bar.