Implementation of `EquivariantPowerSpectrum` and `EquivariantPowerSpectrumByPair` convenience calculators

[jwa7]

Implements:

1. a lambda-SOAP (i.e. EquivariantPowerSpectrum) calculator that wraps DensityCorrelations
2. a CorrelateTensorWithDensity calculator that performs a single CG tensor product between a tensor of arbitrary body order and a density (body-order 2) tensor, where the samples between the tensor and density are different.
3. a EquivariantPowerSpectrumByPair calculator, which generates a SphericalExpansion and SphericalExpansionByPair, manipulates the metadata appropriately, then uses CorrelateTensorWithDensity calculator to perform rho_i x g_ij to generate an Equivariant power spectrum by pair

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📚 Documentation preview 📚: https://rascaline--317.org.readthedocs.build/en/317/