Added exogenous discount to OptionletStripper1 (thanks to Peter Caspers).

git-svn-id: https://quantlib.svn.sourceforge.net/svnroot/quantlib/trunk/QuantLib@18456 8618b1d8-e22c-0410-b026-96a5dba3e089
Increased precision in tail of West's bivariate cumulative normal.

Thanks to Fabien Le Floc'h.

git-svn-id: https://quantlib.svn.sourceforge.net/svnroot/quantlib/trunk/QuantLib@18457 8618b1d8-e22c-0410-b026-96a5dba3e089