Fixed bug, power_law_extrapolation parameter A scaling

Dear All,

I found that "power_law_extrapolation" was giving bad results: the extrapolated tail intensity was multiplyed by some unknown and *huge* factor :). It seems the proble is related with A parameter of the PowerLaw component, that is now scaled by the energy spacing in the channels of the spectral dimension. I guess the easy solution is just to multiply A by this spacing in the function "power_law_extrapolation". 

I've implemented this solution in this pull request, but I'm not sure of the behavior of the A parameter in all cases.

Best wishes,
Alberto.