Estimate Gaussian width from normalized derivate

[jtgasparik]

The components of this PR include:

1. Added function to compute the Gaussian width ($\sigma$) using weighted least squared for the subset of data points in the normalized derivative that correspond to the smallest $d^{2}$ value.
2. Fixed the unit issue so that all time units are in $\mu s$ as aligned with Monteki and Kondo (2008).
3. Modified tests to use newly incorporated PSL dataset in arm-test-data. This dataset provides clean scattering signals better suited for code testing.
4. Added function to plot the $I'/I$ line from associated $\tau$ and $\sigma$ parameters.

[rcjackson]

@jtgasparik I added a commit to your PR adding arm-test-data to the document build environment since that was failing. This was a hiccup on my end so I went ahead and put that in. You should pull that change to your branch accordingly.

[rcjackson]

Once the unit tests pass, I am good with this PR.