Polylogarithm 6.14.0

Changes

• use log1p(x) instead of log(1 + x) to avoid a potential catastrophic cancellation (has not been observed)
• remove unnecessary calls of std::abs and fabs
• replace std::pow(2.0, n - 1) by std::exp2(n - 1) which is faster on some systems
• update doctest.h to 2.4.9