Alternative Duration

[alecloudenback]

The functionality works, but a few unresolved threads:

• The new functions only take vectors of Cashflows instead of the two argument cashflows,times allowed elsewhere in the package
• Should be accompanied by a docpage illustrating the different duration measures and the continuous nature of the new measurement
• Unclear how the new measure would be able to be determined for interest-sensitive cashflows

Some overlap with the issues/thoughts discussed at JuliaActuary/FinanceCore.jl#15