Implement convenience calculator `EquivariantPowerSpectrum` #548

Job	Run time
Lint Rust code (stable, x86_64-unknown-linux-gnu)	12s
check examples / benchmarks	56s
ubuntu-22.04 / rust stable / static C library	2m 31s
ubuntu-22.04 / rust stable / release valgrind	2m 31s
ubuntu-22.04 / rust 1.65 / cmake 3.16	2m 12s
macos-14 / rust stable	2m 32s
windows-2019 / rust stable	2m 20s
	13m 14s

Provide feedback