Minor: Add comments and clearer constructors to Interval

[alamb]

Which issue does this PR close?

related to #7467

Rationale for this change

While reviewing #7467 I found I had to remind myself several times what open/closed meant in the context of intervals.

What changes are included in this PR?

1. Clarify comments on Interval (they were already nice, just could be somewhat clearer)
2. Add Interval::new_open and Interval::new_closed as a way to make the code more self documenting
3. Change some uses of Interval::new to Interval::new_open and Interval::new_closed

I am happy to change more uses but I wanted to see what people thought of the idea first

Are these changes tested?

Existing tests

Are there any user-facing changes?

No functional change intended, just easier to understand code

[wjones127]

These seems good. I would also add: change get_datatype to just datatype to be more idiomatic. (Nevermind, didn't realize get_datatype() returned an owned DataType.)

The other change I was wondering about is whether Display could be implemented to output the set notation representation. Would be cool to get something like [10, 20], [10, ∞), (-∞, 100] or (-∞, ∞) back. But if we don't feel like that's idiomatic Rust, we should at least change the representation so it indicates whether the bounds are open or closed. Right now the Display implementation for IntervalBound just uses value.

[ozankabak]

This looks good to me. I also like the display suggestion @wjones127 👍

[alamb]

The other change I was wondering about is whether Display could be implemented to output the set notation representation. Would be cool to get something like [10, 20], [10, ∞), (-∞, 100] or (-∞, ∞) back. But if we don't feel like that's idiomatic Rust, we should at least change the representation so it indicates whether the bounds are open or closed. Right now the Display implementation for IntervalBound just uses value.

I have filed #7547 to track this suggestion

@ozankabak and @wjones127 if you think this PR is ok to merge as is, can you please github approve it? If not, can you explain what else you would like to see?

Thank you!

[ozankabak]

LGTM, thank you