primesieve-11.2

This is a new maintenance release, it is fully backwards compatible with the previous release. This release contains one CMake bug fix, documentation improvements, tests have been ported to GitHub Actions and the nth prime code has been cleaned up.

• nthPrime.cpp: Rewritten using more accurate nth prime approximation.
• nthPrimeApprox.cpp: Added logarithmic integral and Riemann R function implementations.
• cmake/libatomic.cmake: Fix failed to find libatomic #141.
• .github/workflows/ci.yml: Port AppVeyor CI tests to GitHub Actions.
• doc/C_API.md: Fix off by 1 error in OpenMP example #137.
• doc/CPP_API.md: Fix off by 1 error in OpenMP example #137.
• Vector.hpp: Rename pod_vector to Vector and pod_array to Array.
• iterator.h: Improve documentation.
• iterator.hpp: Improve documentation.
• C_API.md: Add SIMD (vectorization) section.
• CPP_API.md: Add SIMD (vectorization) section.
• README.md: Add C & C++ API badges.

Thanks to @sethtroisi and Sven S. for being primesieve sponsors in this release cycle!