Add support for dynamic properties calculation

[fedluc]

Problem Statement

Currently, the code does not support the computation of dynamic properties such as the intermediate scattering function and dynamic structure factor, despite the availability of well-established formulas in the literature (see reference).

Proposed Solution

We should extend the code to allow for the computation of these dynamic properties across all schemes. This will require:

• Input Modifications:
  • The user must be able to specify the grid for imaginary time, which is necessary for computing the intermediate scattering function.
  • Modifying the input class of the Rpa class should be sufficient to incorporate this functionality.
• Implementation of Dedicated Methods:
  • While the classical and quantum schemes rely on equivalent formulations, their expressions differ slightly
  • Dedicated methods may be required to properly compute the intermediate scattering function for both formulations.

By implementing these changes, we can provide users with a more comprehensive toolkit for analyzing dynamic properties, expanding the applicability of the code.