10920: Extend gammaIncomplete to cover large a≈x

[tedgin]

This addresses #10920.

Adapted the existing function igammaTemmeLarge os that it supports gammaIncomplete as well as its complement gammaIncompleteCompl. As part of this adaptation, log(sigma+1) was replaced with log1p(sigma), since Temme's alorithm requires a to be close to x, which means that sigma is close to 0, making log(sigma+1) imprecise. gammaIncomplete was then modified to use igammaTemmeLarge when x is much larger than 1 and a is near x.

[dlang-bot]

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