CallBond: Add yield to worst calculations

[bondgeek]

bg/instruments/callbond.hpp &.cpp

Add support for Yield to Worst calculations.

• given yield, calculate price based on yield to call for the call that gives the worst price
• given price, calculate yield to call for the call the gives the worst yield.
• iterate through call schedule for all declining calls (yield to worst date will be shortest date with the same price)