Topic/reqs 609

* Demo tests are EquityOptionE2ETest and EquityIndexOptionE2ETest
* Changed EqyOptBjerksundStenslandPresentValueCalculator such that equity option is computed by the same logic. 
* Created EquityOptionBlackScholesThetaCalculator because the existing calculator, EquityOptionBlackThetaCalculator, call the driftless method. 
* Added early exercise condition to Bjerksund-Stensland model. 
(greeks[3] + greeks[4])/100 - write an implementation note here to explain that the standard rho assumes that the yield, y, is fixed, so since b = r-y we must include the second term