pymodulepackage (schueltke): fix for tool_level access

Problem:

• when tool_level in aircraft exchange file is accessed by a module but is not existing, the pymodulepackage throws an error as it can be seen and discussed in weight_and_balance_analysis: aircraft-design!31 (comment 2743760) Solution:
• implement a handling procedure how to continue, when tool_level access failed and set tool_level to 0 in that case

Test:

• test previous version using weight_and_balance_analysis with an aircraft exchange file in a first loop, when tool_level is not yet written in the node masses_cg_inertia and compare it with an execution when the attribute was already written by weight_and_balance_analysis
• test the same procedure with the fix from this MR