primecount-7.10

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

• cmake/OpenMP.cmake: Improve libatomic detection: kimwalisch/primesieve#141
• .github/workflows/ci.yml: Port AppVeyor CI tests to GitHub Actions.
• RiemannR.cpp: Fix potential integer overflows in Li_inverse(x) & Ri_inverse(x).
• nth_prime.cpp: Use new nth_prime_approx(n).
• Vector.hpp: Rename pod_vector to Vector and pod_array to Array.
• README.md: Add C & C++ API badges.
• Update to latest primesieve-11.2 library.

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