Fix FastCaloSimExtrapolation FPEs seen in ATLASSIM-6117

Increases the threshold for bounds in the FastCaloSimExtrapolation to be considered identical to 4x the step size, which should ensure that nHalfDivisions >= 2. Also returns now sets the PCA as averaged quantity (which increases precision, but likely to negligible amount). Should fix the related FPE seen in ATLASSIM-6117.