Add support for explicit mode-by-mode waveform generators

[acorreia61201]

This PR adds generators for generating the ringdown models mode-by-mode, as well as allowing for multimodal waveform plugins that explicitly calculate individual modes to be generated in the same way.

Earlier iterations of PR #5229 attempted to calculate likelihoods by generating templates as individual modes (e.g. QNM approximants). The current iteration of that PR no longer requires these changes; however, I've kept them here in case they end up being useful in the future.

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