Recommend re-licensing from GPLv3 --> LGPLv3

[amykyta3]

It turns out GPLv3 is somewhat inappropriate for distribution of libraries, and I didn't quite understand the nuance when I initially started the PeakRDL project. As a result, many have followed my example and also used GPLv3.

LGPLv3 is nearly identical, but is more appropriate for Python package distribution. I have switched all the core PeakRDL components to LGPLv3, and would recommend the same for PeakRDL-Markdown.

See this PR as an example: SystemRDL/PeakRDL-ipxact#34