Decreasing depths and curve step

The test well `tests/assets/F03-02.las` loads but the depths are reversed (as in the file). The curve step is given as 0 in the file and `welly` determines it to be zero (but I'm not sure why because it seems like [`utils.get_step_from_array()`](https://github.com/agilescientific/welly/blob/main/welly/utils.py#L712-L731) should result in a negative number.