Add minimal input current to pass IShuntA/IShuntD criteria to SLDO analysis

At the requested of the Yield Task Force, we need to add two new fields in prodDB for each chip,
SLDO_MIN_I_PASS_ISHUNTA SLDO_MIN_I_PASS_ISHUNTD

The analysis needs to be expanded to calculate those values. They corresponds to the minimal input current for which the chip will pass the IShuntA/IShuntD criteria. To do that, the software needs to fit the linear part of the IShuntA/D-vs-InputI curve, and extrapolate the fit to identify the crossing point to the cut criteria.

Given that it's unclear how precise is the measurement circuit inside the chip when IShuntA/D are very close to zero, it's important to extrapolate from the linear fit in a region where there is enough shunt current, but before saturation effects start to appear.

Tagging @mmarjano for the changes in prodDB, and @ngruen and @dfellers for the implementation.