[PLAT-6852] Implemented exponential interpolator

* Reimplemented exponential interpolation. 
* Modified InterpolatedDoublesCurveTest because the exponential interpolator is not relevant to the data set in the test. 
Add a test where the data on different sub-intervals is not coming from the same exponential. This is to make sure that the coefficients a and b computed are not computed on one interval and used on another one.