Applies `function` in-place to the field name specified.

        In other words, `mutapply` overwrites column `fieldname`
        ith the results of applying `function` to each element of that column.

def mutapply(self, function, fieldname):
        """
        Applies `function` in-place to the field name specified.

        In other words, `mutapply` overwrites column `fieldname`
        ith the results of applying `function` to each element of that column.
        """
        self[fieldname] = self.apply(function, fieldname)