WIP: SAMP clean-up 2

- [x] Split `vo.samp` code up into multiple files
- [x] Convert docstrings to astropy format
- [x] Run autopep8
- [ ] Rename class methods to follow PEP8 (use `snake_case`)
- [ ] More unit tests
- [ ] More documentation

And maybe these?
- [ ] Replace logger class with astropy logger
- [ ] Use properties where appropriate