Remove pub/sub

[jdavid]

Remove pub/sub

[jdavid]

Done in PR #232

These are follow-up actions:

• Review the list of issues and close those that are now obsolete
• Clean code in caterva2/services/sub.py supporting old roots, instead raise errors
• Change default port from 8002 to 8000
• Rename subscriber to server
• Update documentation