Implement a ci16-based beacon detector

[rdoosty]

Currently, there are some AVX2 functions in comms-lib-avx.cc that try to implement correlation for complex int16, but they are not complete.
static std::vector<std::complex<int16_t>> correlate_avx( std::vector<std::complex<int16_t>> const& f, std::vector<std::complex<int16_t>> const& g);