Compute d2

[jtgasparik]

Function to compute the statistical distance ($d^2$) for an associated value of $\tau$. This PR contains:

1. Added helper functions for the NDM module which consolidates the overlapping computations between the $\tau$ and $d^2$ functions.
2. Function to compute the $d^2$ value used to determine the best $\tau$ value in the non-evaporative region.
3. Tests to compare the calculated $\tau$ value based on the best $d^2$ value and compare to true values from purely scattering particles.
4. One test to compare the best $\tau$ value for an absorbing particle and compare to the true value from the purely scattering particle (this is an approximate way to test because $\tau$ values should not vary significantly within a single population). Testing tolerance slightly higher to accommodate the evaporative particle.
5. Added an optional flag to the plot_normalized_derivative function to plot the scattering signal on the same plot as the normalized derivative for comparison purposes.