Implement the __round__ function

[plexluthor81]

This is a very handy project! I used it as part of a dozenal clock app I wrote, and I needed a round function so that I could show fractional days with limited precision.

I'm not sure that my code is any good (I suspect it might exacerbate numerical errors) but it met my needs, so perhaps you want to include it.

    def __round__(self, ndigits=None):
        if ndigits is None:
            ndigits = 0

        divisor = self.base**-ndigits
        quotient, remainder = divmod(self.value, divisor)
        if remainder/divisor > 0.5:
            quotient = quotient + 1

        dqs = BaseConverter(quotient, digits=self.digits).string
        if ndigits > 0:
            return BaseConverter(dqs[:-ndigits] + '.' + dqs[-ndigits:], digits=self.digits)
        else:
            return BaseConverter(dqs + '0'*-ndigits, digits=self.digits)