Expand unit tests

Even though the coverage is 100%, not all possible cases are covered. For example, we need to check what happens if e.g. an Astropy quantity is passed to a pint NumericalProperty. Or what happens if we pass an array with units to a NumericalProperty with no units.

Start by finding the existing unit tests for NumericalProperty and inspect how Astropy quantities and arrays with units are currently handled. Add coverage for the two example cases and verify that the tests clearly capture the intended behavior for incompatible or missing units.

Written by the indexing model from the issue text.