Change mtheme license

[lambtho12]

The CC-BY-SA 4.0, as any other creative common license is not adapted for software and codes. Creative Commons even explicitly advise AGAINST using their licenses for software in their FAQ.

Instead, they suggest the use of proper open-source licenses, such as the GPL or MIT for instance. For the specific case of latex packages (which is the application here), the recommended license is the LaTeX project public license. Although the lppl does not enforce the same conditions as CC-BY-SA. If these conditions are an absolute necessity, I think GPL would be the best solution.