Add method for fetching specific hour of each day

[krowvin]

In some reports users need an 8 am value but only have hourly data available.

Instead of having to loop and create a new Value() instance for each timestamp at 8 am

I propose we create a method on the Value class that lets you select an hour, minute, day, etc of the datetime object.

For example say you want 8 am values for each day of a months data.

Users could call:

myVar=Value(...)
myVar.setValuesAt(hour=8)

And then this would update the class to only have values at 8am or possibly just print them at this time.