Various fixes and minor Python refactorization

General:

• move config directory to root of repo
• add .codespellrc which is also seen by pre-commit -- should probably move some gitlab.ci options there as well

Refactorization [Python]:

• Monitoring: allow to programmatically add metrics scheduled for polling, improve error handling. Fixes #68 (closed)
• Config: more flexible flattening helper routine, rename module file
• Satellite: make config explicit as payload to initialize, addressing #67
• Controller: add --config argument, allow to supply nested dict as config to initialize which will be ad-hoc flattened for each Satellite. Fixes #60 (closed)
• CDTP: small fixes in CDTPMessage class
• extend tests, keep test cfg separate from example