Frequency-domain susceptibility

[sgkang]

add chi (susceptibility) as an input for GlobalEM1DProblemFD

So, one can pass distribution of chi, e.g.

prob = GlobalEM1DProblemFD(
    [], 
    sigmaMap=mapping, 
    chi=chi, 
    hz=hz, 
    parallel=False, 
    n_cpu=2, 
    verbose=True,
    Solver=PardisoSolver
)

you are good to go! @fourndo

[coveralls]

Pull Request Test Coverage Report for Build 119

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Totals
Change from base Build 114:	0.0%
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💛 - Coveralls