Decay Heat Method

1. adding decay heat method
2. adding constant to convert between MeV/s and MW
For some reason I can't remember I had power on the mind! Energy is more intuitive and common for this conversion, so I'll change it to that

> On Dec 10, 2014, at 9:35 AM, Andrew Davis <notifications@github.com> wrote:
> 
> In pyne/data.pyx:
> 
> > @@ -55,6 +55,8 @@ sec_per_day = cpp_data.sec_per_day
> >  MeV_per_K = cpp_data.MeV_per_K
> >  """Megaelectronvolts per Kelvin."""
> >  
> > +MeV_per_MW_s = cpp_data.MeV_per_MW_s
> > +"""Megaelectronvolts per megawatt-second."""
> Im surprised we dont already have the 1.602e-19 number as a PyNE constant anyhow
> 
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