let the calibration helper observe the evaluationDate and the floating rate index,
move the constructor code to performCalculations(),
provide swaption constructors specifying dates instead of tenors,
allow for non atm strike swaptions and a non unit nominal
remove volatility and nominal inspectors from swaptionhelper
add volatility inspector to calibrationhelper
remove registration with evaluation date in base class, if needed in
derived classes, they should register by themselves
Return the contained handle by value.

This doesn't limit the intended uses but prevents client code from
breaking its link with sister handles.
Newly added performCalculation method set to private.

Having it as public in the CalibrationHelper class was probably an
oversight. Unfortunately, we can't change it there for backward
compatibility.
Allow recalculation of exercise and end dates.

Setting the calculated values to the stored data members would prevent
recalculation if the evaluation date changed. Instead, using temporary
objects and leaving the data members null will trigger recalculation.
Merge pull request #12.

Various enhancements to short-rate calibration helpers.