Add IR and Raman spectrum functionality

Add tools for computing IR and Raman spectra from TNEP dipole and polarizability output, along with the dielectric tensor and Kramers-Kronig relation.

New functionality

• get_ir_spectrum — computes the IR spectrum from a dipole or polarization time series
• get_raman_spectrum — computes the Raman spectrum from a polarizability time series, with support for Porto notation polarization selection
• get_dielectric_function — computes the full imaginary dielectric tensor from P(t) or dP/dt; supports Gaussian windowing, ACF truncation (max_lag), and polarization-projected output (replaces get_dielectric_function)
• apply_kramers_kronig — adds real part of the dielectric function via the KK relation; two implementations (O(n²) vectorized default, O(n log n) FFT variant)
• read_dipole, read_polarizability — readers for GPUMD dipole and polarizability output files
• apply_quantum_correction — quantum correction factor for spectral intensities

Tutorials

• Training a TNEP model for the polarization (tnep_polarization.ipynb)
• Training a TNEP model for the susceptibility (tnep_susceptibility.ipynb)
• Predicting IR spectra (predicting_ir_spectrum.ipynb)
• Predicting Raman spectra (predicting_raman_spectrum.ipynb)
• Computing the dielectric function (dielectric_function.ipynb)

API notes

• All spectrum functions use max_lag (fraction of trajectory length, (0, 1]) instead of an integer sample count to control ACF truncation; defaults to 0.25
• Angular frequency output is in THz throughout