High numbers in /worldborder set and /worldborder add round to incorrect values

High numbers in /worldborder set and /worldborder add round to incorrect values. This is because the command parser uses a float, and it will round higher values, since it cannot represent them. This would be fixed by using a double instead. (All other position-related arguments also use doubles)

How to reproduce:
1. Set the world border size to a smaller value (/worldborder set 10)
2. Set the world border size to 59999998 (/worldborder set 59999998)
3. It will show up in chat as 60000000 (it rounded to the wrong number)