Add floor() and ceil()

[wsbankenstein]

Description

Add floor() and ceil() rounding functions to simplexpr.
No additional dependencies or code changes.

Checklist

Please make sure you can check all the boxes that apply to this PR.

• [V] I added my changes to CHANGELOG.md, if appropriate.
• [V] The documentation in the docs/content/main directory has been adjusted to reflect my changes.
• [V] I used cargo fmt to automatically format all code before committing

[elkowar]

Your current docs suggest that the floor and ceil functions take two arguments, whereas your implementation seems to only take the actual number (no decimal-digit count). I'd suggest we add both cases, so it defaults to flooring to 0 decimal points, but if an additional argument is provided, those get considered as well

[wsbankenstein]

Yeah, I removed the extra argument but forgot to update the documentation. I don't think an optional second argument is needed, because that functionality can be achieved easily with round(floor(num), digits). Also, it's not the most necessary functionality to have, seeing as all the digits after the decimal point would be zeroes.

Documentation is now updated.