Add option to specify output folder

[Victor-Savu]

At the moment, gleam outputs the fit results and plots next to the corresponding input spectra.

It would be great to have the options to output the results to a different folder. This can help:

• speed up the execution when the input data is hosted on a slower drive (like a network drive).
• back up the results without having to pick them out from among the data.
• produce comparable & archivable snapshots of different runs with different input configurations. Maybe the input configuration could be copied or somehow linked to the output folder in order to help with keeping track of the results.