Topic/plat 6820

[PLAT-6820] Generic Implied Volatility Solver
[PLAT-6821] Bjerksund-Stensland model fails for zero volatility

[PLAT-6820] exposed [PLAT-6821] - which is it part of the same pull request. There is also a debt item [PLAT-6822] to further review Bjerksund-Stensland.