position at which to evaluate.
full width at half maximum of the lower-x half.
full width at half maximum of the higher-x half.
the value fct and its partial derivatives with respect to x (dx), fwhmLow (dFwhmLow) and fwhmHigh (dFwhmHigh).
Analytical value and partial derivatives of the split gaussian function centered at x=0. Each half's fwhm only affects its own side, so the off-side derivative is 0.