Extend tests for plasma_dispersion_func
Extend tests for plasma_dispersion_func

I added a test against a power series expansion that I think is valid
around 0 + 0j, and added tests for the first five zeros of the panda
dispersion function.

I increased the accuracy of many of these tests by using mpmath to do
this at greater precision:

    from mpmath import j, sqrt, exp, erf, findroot
    def Z(w):
        return j * sqrt(pi) * exp(-w**2) * (1 + erf(j * w))

To find the roots at greater precision than in the appendix of Plasma
Waves by Swanson (2nd edition), I did:

    findroot(Z, 4.106_107_271 - 3.725_948_279j)

StanczakDominik/PlasmaPy