Improve logging

[jojomale]

Problem

• some modules do not log at all
• log-handlers accumulate if Correlator is called repeatedly, e.g. in a Loop, leading to cluttered output
• log messages could be more specific

Proposed solution

• introduce logging across several modules as described in https://docs.python.org/3/howto/logging-cookbook.html#using-logging-in-multiple-modules
• on initialization, check if handlers are already present before adding
• specify log messages