Switch PyPOTS ecosystem license from GPL-v3 to BSD-3-Clause

[WenjieDu]

Issue description

When I created PyPOTS, my thought was simple: open-sourcing some useful SOTA algorithms with quality code. Quality code is so important to open-source communities, and I expect projects using PyPOTS to be open-source as well. Therefore I applied GPL-v3 license in PyPOTS.

Now, more and more people are noticing this project and using it in their research and engineering work. Also, our community is always trying to make PyPOTS better. Considering the infectious nature of GPL licenses that could make a small part of our users concerned, I think PyPOTS and its ecosystem projects should all switch to using BSD license, and this should be done synchronously in their version 0.2 release.