Age in month and year are not equivalent to fixed-day interval

[hugy718]

Problem

The current age calculation for week/month/year is based on translation to the fixed day interval:

COOL/cool-core/src/main/java/com/nus/cool/core/cohort/utils/DateUtils.java

Lines 70 to 75 in 80070ff

	case MONTH:
	return new TimeWindow(d.toDays() / 30, TimeUtils.TimeUnit.MONTH);
	case WEEK:
	return new TimeWindow(d.toDays() / 7, TimeUtils.TimeUnit.WEEK);
	case YEAR:
	return new TimeWindow(d.toDays() / 365, TimeUtils.TimeUnit.YEAR);

Needs to use the date object to generate the difference directly and add tests to cover these cases.

This is related to issue #134, PR #135