Expand interp cart grid testing

[djps]

This extends the tests for ìnterp_cart_grid` so that different sensor data is stored at different positions.

[waltsims]

Related to #572

[djps]

I have no idea why ruff/isort is not working, although it doesn't matter too much.

Have have increased the test coverage for interp_cart_data,

[djps]

ok, i have absolutely no idea why ruff/isort fails and although it is not important, i would like to know why.

As for the test coverage - I am not sure where the motifications of interp_cart_data come from, but there are tests which help maintain the code coverage but don't actually ensure the code is tested, for example test_interp_cart_data_2_points_linear in test_utils.py. I had put a pytest.skip pragma in to stop them running as it is gives a false impression.