Code generation improvements

[feltech]

What

Following on from discoveries and discussions as part of #11

Tasks

• [Tests][C++][Python] Test copyright headers added to all generated files
• [Tests][C++][Python] Add missing tests of hoisting header/module docstrings
• [Tests][C++][Python] Ensure long docstrings are wrapped appropriately
• [Core][C++][Python] Choose/create exception type(s) raised when trait property not available
• [Core][C++][Python] Factor out common utility code
• [Tests][Python] Test behaviour when requesting trait property of non-imbued trait
• [Core][Python] Validate module names
• [Core][Python] Do not duplicate "Conforming" warnings
• [Core][Python] Remove usage of TraitBase and SpecificationBase #19
• Improve usage info in docstrings  #10