Harmonise the MID and LOW SensCalc backends

The backend of the Sensitivity Calculator currently takes a very different approach for MID and LOW telescopes. LOW is a purely function based calculator, whereas MID is object based. This makes maintaining the repo and sharing code between the MID and LOW telescopes more difficult.

This features proposes the work necessary to harmonise the MID and LOW calculators. This includes:

• Converting the MID calculator to be function based, rather than object based.
• Converting the LOW calculator to use AstroPy units.
• Enabling some basic validation by implementing pydantic models for the CalculatorInputs.