Simplify debug level

Currently a debug level is set in logger and as parameter to propulatur or islands.
Should be set only in one place.

Also the parameter sometimes sets verbosity at higher levels adds expensive checks.
Move the checks to tests and name the parameter verbosity or just be consistent and descriptive.

Start by tracing how the logger and the Propulator or Islands entry points currently receive and use the debug-level parameter. Identify the checks that change verbosity or add expensive work, then review the existing tests to determine where those checks should be covered. Done means one consistent, descriptive verbosity setting and tests covering the intended behavior.

Written by the indexing model from the issue text.