LeBailFit is slowed down by calling E1()

[wdzhou]

This issue was originally TRAC 7900

Back-to-back exponential convoluted by pseudo-voigt calls function E1() do a contour integral when the Lorentzian contribution in pseudo-voigt is not zero. It severely slows down the speed of LeBailFit(). Try to figure out some solution to speed it up.

[wdzhou]

@NickDraper (2014-02-14T11:05:00):
Bulk move to assigned at the introduction of the triage step

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