Add parallax uncertainty

[aggregate1166877]

This project's processing scripts were initially written in such a way that parallax uncertainty is discarded at the end of data processing.

Since then, it's been noted by a user on StackOverflow that parallax uncertainty is actually useful in some contexts.

Task:

• For stars taken from Simbad, keep parallax information.
• For amended stars, open all reference links, find parallax uncertainty, and add it to the amendment fields.

The processing script (buildRaDecJson.js) should store uncertainty values in bsc5p_spectral_extra.json.