Topic/plt 401

Create a LastTimeIndexCalculator which specifically overrides the default LastTimeCalculator behaviour for basis swaps to use the base leg to compute the curve offset, rather than the max of the base and basis leg.
USDLIBOR1M, USDLIBOR3M, USDLIBOR6M, GBPLIBOR6M, DKKCIBOR6M, JPYLIBOR3M strings should reference the new static strings created above for use in the _iborIndexMaster.getIndex("xxxxx") methods.