(feat) optional mass weighting in VACF

[jwjeffr]

Summary

This PR adds an optional mass weight option to the VACF calculation in properties.correlations.VelocityAutoCorrelation, similar to the implementation in pwtools, which is necessary for getting accurate spectra in multicomponent systems.

Feature

• Added mass weighting as a Boolean flag which defaults to False for backwards compatibility. I.e., if the user passes mass_weighted=True into the initializer, then the VACF is:

$$ C(t) \propto \sum_i m_i \left\langle \mathbf{v}_i(0)\cdot\mathbf{v}_i(t)\right\rangle$$

rather than the unweighted case where $m_i = 1$. The main changes are here, and are similar to the implementation in pwtools here.

Checklist

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• Doc strings have been added in the Google docstring format.
• Run ruff on your code.
• Tests have been added for any new functionality or bug fixes.