oc 小数乘法，并取整

oc的double，float类型无法满足小数乘法的需求

只能用NSDecimalNumber实现

    NSDecimalNumberHandler *roundingBehavior = [NSDecimalNumberHandler decimalNumberHandlerWithRoundingMode:NSRoundDown scale:0 raiseOnExactness:NO raiseOnOverflow:NO raiseOnUnderflow:NO raiseOnDivideByZero:YES];

 NSDecimalNumber *a = [[NSDecimalNumber alloc]initWithString:@"0.8"];
    NSDecimalNumber *b = [[NSDecimalNumber alloc]initWithString:@"0.6"];
//相乘并取整
    NSDecimalNumber *c = [a decimalNumberByMultiplyingBy:b withBehavior:roundingBehavior];