CLN: clean IntervalArray._simple_new
There is a lot of code in `IntervalArray._simple_new`. I suggest moving all the code there that checks that the input data is correct into its own method (`_ensure_simple_new_inputs`), so we have a better split between building the ÌntervalArray` and checking that its inputs are correct.

This PR changes no functionality.
The arm failure looks unrelated. I've updated the PR. thanks @topper-123 