[PLAT-6852] Implemented exponential interpolator

* Reimplemented exponential interpolation. 
* Modified InterpolatedDoublesCurveTest because the exponential interpolator is not relevant to the data set in the test. 
Final result can be written directly (without computing the intermediary values:
return y1 * Math.pow(y2 / y1, (value - x1) * xDiffInv); (remove lines 55 and 56).
This should save one log and one exp computation.

firstDerivative and getNodeSensitivityForValue should probably be adapted also.