Cache spectra (`gls` and `cls`)

[Saransh-cpp]

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In a separate step, let's also open an issue on how/whether to cache the spectra in our examples. This could in and of itself be a useful example; I cache spectra (actually the gls, not just the cls) in almost every simulation I run.

Originally posted by @ntessore in https://github.com/glass-dev/glass/pull/345#pullrequestreview-2369140490