feat: Add round_sig_figs expression for rounding to significant figures

[owrior]

Recently I have been working on a project where we needed to round to significant figures and realised there is no way to do this in pandas, numpy or polars. I found this as the best solution.

def signif(x, p):
    x = np.asarray(x)
    x_positive = np.where(np.isfinite(x) & (x != 0), np.abs(x), 10**(p-1))
    mags = 10 ** (p - 1 - np.floor(np.log10(x_positive)))
    return np.round(x * mags) / mags

However it would be nice to have it as part of a package. Would it be possible to include this? (The implementation here is about 20% faster than the numpy one here already but I imagine there is a better way of implementing it) (Update now about 45% faster).

Thank you!

Closes #11968

[stinodego]

Hi @owrior, thanks for the PR. Could you please make an issue so we can discuss whether we want to include this, and what the right API for it would be? Thanks!

From a quick initial look, the boilerplate here looks quite good 👌

[owrior]

Refactored to round_sig_figs @orlp so think this might be ready!

[orlp]

Could you use f64::powi? Also it's spelled magnitude.

[owrior]

Think this is done.

[stinodego]

I will do a small update to the docstrings, will update and merge after.