Export labels into trx.mat files

Manual and automatic labels are currently exported to JSON files.

Problem: Existing analysis tools take trx.mat files as input.

Proposal: Allow exporting into trx.mat files as well; labels only, as named columns of {-1,1}.

Note that the new .mat files may exhibit some additional differences, as they will be written using libraries such as MAT.jl instead of MatLab. Some tests should idealy involve some of the above-mentioned analysis tools, but it's unlikely such tests are ever included in the current project.

EDIT: See https://gitlab.pasteur.fr/nyx/planarlarvae.jl/-/issues/2.