LICENSE added, GNU Lesser General Public License v3.0

@sly2j Before we can have any GSoC student work on Juggler algorithms, we need to pick a license. I'm just going to start the discussion by creating a MR for the Apache License 2.0 (which Gaudi is licensed under). Key4hep has no license for their framework yet, but it may be useful to start that discussion with them.

We would need to get agreement from all contributors before we can merge this (or remove their contributions), assuming they own the copyright to their own code.