Allow f_E to be specified in terms of widths _or_ power
Allows the feedback energy fraction to be specified in terms of the width of the transition or the power law index.
Helpful for emulation, as we don't want complex transformations between input variables and those actually used in the code.
Also improved slightly the printing of the EAGLE feedback model.
I'm using FLT_MAX as a temporary power law index in the case where we don't find anything. I presume there is a better pattern to use here so I'm open to suggestions.
@dc-bahe1 is likely to be interested in this.