Fretlabel

- Restructure project to use the source layout
- PEP517: Manage project with Poetry as the build backend
- Rename the project to *FRETlabel*
- Migrate photon sampling module to *FRETraj*
- Update docs and tutorials to Juypterbook

**Full Changelog**: https://github.com/RNA-FRETools/fretlabel/compare/v0.1.0...v0.2.0

Added
- first release of fluordynamics including the PyMOL plugin *Fluorlabel* and the Monte-Carlo burst sampling algorithm *Fluorburst*