fix: don't overwrite user-configured logging levels

[psschwei]

Fixes #214

Fixes an issue where user-configured log levels on fancy_logger were being overwritten when MelleaSession was initialized. The fix checks if the logger level is still NOTSET before applying the default level, preserving any level the user set beforehand.

[mergify]

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[nrfulton]

Thanks, looks good. We're in code freeze until #310 is merged. That should resolve automatically for this PR. I will merge this some time tomorrow (after #310), or ping you if there are unanticipated merge issues with #310.

[nrfulton]

#310 is now merged, which created conflicts (we moved fancy_logger, I think). Because this PR is from a branch on your repo, it'll be easiest if you resolve the merge conflict and push. Once it's resolved I will run the CICD checks and merge. Thanks for your patience.