Excoupon and payment calendar parameters for CPI Bonds

Implement excoupon parameters and payment calendar for CPIBonds, similar to what is already done in FixedRateBonds
For backward compatibility, this should still be used if `withPaymentCalendar` is not used, as in:

    if (paymentCalendar_.empty())
        paymentCalendar_ = schedule_.calendar();

so that old code using `CPILeg` doesn't break (in fact, there are four failures in the test suite on this branch, and I suspect this is the reason).