improved NumDurationExtensions to make it more readable

[himanshugarg08]

When I was looking for the code of NumDurationExtensions, have to do a lot of computation and figuring out what 1000 or 60 is for what purpose

So, Tried to reduce it in more readable format.

For example: 1 day = 24 hours {someNum}.days = someNum * 24 hours (this can be written as ({someNum} * 24).hours)

Similarly, for other extension also.

and .microseconds can take care of .round() at last.

Let me know what do you think? Or that microseconds is used deliberately?