a `keep_left` option

[zkurtz]

This request is possibly a special case of #11, if you consider the case of "0 decimals".

Typically my main purpose for using significant figures is to avoid using more digits than I need to use to display a value. With that in mind, an operation like round(1234, 2) -> 1200 has no value. "1200" has the same number of digits as "1234". I don't find 1200 necessarily any easier to read, at least if I apply a spacer like "1,234", and it contains strictly less information than 1234.

So what I'd like to have is a keep_left option, meaning "keep all digits that are to the left of the decimal point":

round(1234, 2, keep_left=True) -> 1234
round(123.4, 2, keep_left=True) -> 123
round(12.34, 2, keep_left=True) -> 12
round(1.234, 2, keep_left=True) -> 1.2

It's easy to implement this; I currently use something like

def sigfig_keep_left(num: float | int, sigfigs=2) -> str:
    """Format a number with a given number of significant figures.
    
    Any rounding applies only to decimal digits, not to the number of digits in the integer part.
    """
    num_str = str(num)
    num_nondecimal_digits = len(num_str.split(".")[0])
    ndigits = max(sigfigs, num_nondecimal_digits)
    return sround(num_str, sigfigs=ndigits, spacer=",")

I'd like to PR this into sigfigs in some form if that functionality would be accepted, please advise!