Feature: Downloading spectra of individual conformers

[danielhollas]

Currently, the Download spectrum button on the SpectrumWidget downloads a csv file that only contains total cross section.
We need to add cross-sections of individual conformers as well so that they can be plotted externally.

This should be implemented in the _prepare_payload() method of the SpectrumWidget.

aiidalab-ispg/aiidalab_ispg/app/spectrum.py

Line 361 in 2c17cf9

# TODO: Download multiple spectra if available

The conformer spectra should be accessible with something like:

        f = self.figure.get_figure()
        y_conf = []
        for conf_id in range(n_conformer):
            label = f"conformer_{conf_id}"
            line = f.select_one({"name": label})
            y_conf.append(line.data_source.data["y"])

y_conf array should then be added to the writerows() call

     writer.writerows(zip(x, y, *y_conf))