PLT-426 - TimeSeriesRelativeWeightedDifferenceOperator does not handle weights of 0

Add code and test to handle weights of zero
PLT-426 - TimeSeriesRelativeWeightedDifferenceOperator does not handle weights of 0

Fix typo in method name
Merge pull request #195 from OpenGamma/topic/PLT-426

PLT-426 - TimeSeriesRelativeWeightedDifferenceOperator does not handle w...