Initializes greeks to null values and cheks vanilla values in fwd evaluation.

Greeks and MoreGreeks aren't initialized on construction, leading to errors when using quanto and forward engines.
Besides, forward engine doesn't check greeks results, returning bogus values when the vanilla engine can't calculate some greeks.
added test for catching missing initialization
Remove empty constructors.

We can safely rely on the default-generated ones.
Merge pull request #139.