FastDispersionFitter import data format support

[Frank-DingYC]

Current

We only support the following data format for FastDispersionFitter:

• $n(\lambda)=n+jk$
  • $\lambda$: wave length

Expect

We can support the following data format for FastDispersionFitter:

• $n(f)=n+jk$
  • $f$:frequency
• $\epsilon(f)=\epsilon_{real}+j\epsilon_{imag}$
  • $\epsilon_{real}$: real part of the permitivity
  • $\epsilon_{imag}$: imaginary part of the permitivity
• $\epsilon(\lambda)=\epsilon_{real}+j\epsilon_{imag}$
• $\epsilon(f)=\epsilon_{real}+j\epsilon_{real} \cdot tan\delta$
  • $tan\delta$: loss tangent, which has the relation $tan\delta=\frac{\epsilon_{imag}}{\epsilon_{real}}$
• $\epsilon(\lambda)=\epsilon_{real}+j\epsilon_{real} \cdot tan\delta$

[weiliangjin2021]

The rest waiting until we have frequency-wavelength conversion convenience function? @tylerflex