Subscribe x, y and z spin component latex labels

The purpose of this MR is to make the x, y and z subscribed in the spin component latex labels. This fixes issue #220 (closed).

This is ready to be merged (subject to passing CI).

The CI is expected to fail because of bilby#489 (closed). However, if it passes everything else, this can be merged.

approved this merge request

mentioned in commit facdfa0d

merged