Prepare new release

[juhasch]

• Introduce fractdict to handle fractions in units.
• Refactor arithmetic operations (*, /, //) in PhysicalUnit and PhysicalQuantity for type consistency and Python 3 compatibility.
• Resolve circular imports between unit.py and quantity.py via local imports.
• Correct logic in PhysicalUnit.is_power and dBQuantity comparison methods for proper dimensional handling.
• Fix PhysicalUnit.__hash__ to enable unit/quantity hashing and restore @lru_cache on findunit.
• Improve error handling and type checks in findunit and add_composite_unit.
• Correct unit handling and type checking in numpywrapper.linspace.
• Update tests to match corrected behavior and expected exceptions.
• Improve code readability (f-strings, type checks).