MEI: Calculate the time to burn down all the defects backlog

[jensstutte]

Based on this doc.

Estimate the time needed to burn down the defects backlog based on the past velocity.

If WeighedClosedDefects(TimeInterval) > WeighedOpenedDefects(TimeInterval):
    WeighedBurndownTime(TimeInterval) := Duration(TimeInterval) * (WeighedOpenDefects(Now) / (WeighedClosedDefects(TimeInterval) - WeighedOpenedDefects(TimeInterval)))
Else:
    WeighedBurndownTime(TimeInterval) := Infinity

If NumClosedDefects(TimeInterval) > NumOpenedDefects(TimeInterval):
    NumDefectsBurndownTime(TimeInterval) := Duration(TimeInterval) * (NumOpenDefects(Now) / (NumClosedDefects(TimeInterval) - NumOpenedDefects(TimeInterval)))
Else:
    NumDefectsBurndownTime(TimeInterval) := Infinity

We calculate two variants to compare the difference between weighed and unweighed defect numbers as it is not clear if there is a strong relationship between "being impactful" (high severity) and "being expensive" (much effort to be fixed/closed).