verify accuracy of fractal S(q) for small q#722
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The equations for the fractal S(q) model were complex enough that it was not immediately obvious how they would behave at small q.
This PR rearranges the equations to make the limiting cases much more apparent, and checks them against the original equations from Teixeira (1988) using 500 bit precision.
The numpy version of the equations was added to sasmodels.special so it will be available for pure python models.