Exporting results to .mat from the GUI

[cncastillo]

I think it would be handy to be able to easily export .mat files directly from the GUI (for the signal and recon). An additional button can be added to the Plotly.js plots, but to map it to a MAT export GUI is something that needs to be researched further.

• The signal exporter must include the raw data, the k-space trajectory, and the simulation dict.
• The recon exporter must include the reconstructed image and the recon dict.