Improve testing workflow to select correct environment and handle pandas / python version for map function

[bastigw]

Hi Jia,

this pull request introduces improvements to both the testing workflow and the codebase to enhance compatibility across development environments.

Testing workflow updates:

• Updated the notebook test command in .github/workflows/test-notebooks.yml and pytest.ini to use the --nbval-current-env flag, ensuring that notebook tests run in the current Python environment rather than a potentially isolated one.

Pandas / Python version compatibility:

• Refactored numeric conversion logic by introducing a backward_compatible_map function to handle differences between pandas versions (map for >=2.1.0, applymap for older versions). This is similar to your commit last week, but should improve maintainability.

Minor bug fix:

• Simplified the conversion of phase values to radians in unitconverter by removing unnecessary type casting, relying on prior numeric conversion.