Add debug logging to ModelicaSystem sendExpression

[ondras12345]

I have just spent several hours debugging why ModelicaSystem() cannot load my custom package.
It turns out that it does not support relative paths, because it cd()s to a tmp directory.
However, that is really hard to see without appropriate logging.

This PR allows the user to see exactly what's happening by enabling debug logging:

import logging
logging.basicConfig(level=logging.DEBUG)
logging.getLogger("OMPython").setLevel(logging.DEBUG)

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[lochel]

Very good 👍