### Briefly, what does this PR introduce?
This introduces the LGPL for the geometry description and plugins, as
already used by juggler and in EICrecon, as already attempted in #50 but
delayed due to other activity. We need positive approval by every
contributor (i.e. please check the box next to your name):
- [x] @wdconinc
- [x] @c-dilks
- [x] @veprbl
- [x] @sly2j
- [x] @ShujieL
- [x] @mposik1983
- [x] @rymilton
- [x] @nschmidtALICE
- [x] @johnlajoie
- [x] @rahmans1
- [x] @yezhenyu2003
- [x] @lkosarz
- [x] @mariakzurek
- [x] @ajentsch
- [ ] ~~@faustus123~~ refused; contributions reverted
- [x] @shimasnd
- [x] @jizhongling
- [x] @johnny8266
- [x] @jiheekim222
- [x] @demisra
- [x] @simonge
- [x] @whit2333 
- [x] @Chao1009
- [x] @drateots 
- [x] @adamjaro
- [x] @dhevang 

### What kind of change does this PR introduce?
- [ ] Bug fix (issue #__)
- [x] New feature (issue #__)
- [ ] Documentation update
- [ ] Other: __

### Please check if this PR fulfills the following:
- [ ] Tests for the changes have been added
- [ ] Documentation has been added / updated
- [x] Changes have been communicated to collaborators

### Does this PR introduce breaking changes? What changes might users
need to make to their code?
No.

### Does this PR change default behavior?
Yes, the cmake option `USE_DDG4` was removed.

Co-authored-by: Dmitry Kalinkin <dmitry.kalinkin@gmail.com>
Co-authored-by: Christopher Dilks <c-dilks@users.noreply.github.com>