Update C# NQuantLib project file
Update C# NQuantLib project file

Adding InterestRateVector.cs created by PR#22 to the NQuantLib C# project
file.

fabrice-lecuyer/QuantLib-SWIG,fabrice-lecuyer/QuantLib-SWIG,fabrice-lecuyer/QuantLib-SWIG,fabrice-lecuyer/QuantLib-SWIG,fabrice-lecuyer/QuantLib-SWIG,fabrice-lecuyer/QuantLib-SWIG,fabrice-lecuyer/QuantLib-SWIG,fabrice-lecuyer/QuantLib-SWIG,fabrice-lecuyer/QuantLib-SWIG