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. 
Again, refactor this to a private method