To detect weak signals from reflected GPS L1 signals from the ocean
1ms coherent integration is not enough. We need 1000x1ms noncoherent
integrations to get enough processing gain

Currently not working for all signals because of code doppler correction
and indexing bug

Scalar downconvert! function is extremely slow compared to SIMD
vectorized downconvert! function. Its better for the program to fail
with missing function rather to run extremely slowly

Some RF frontends (such as TDS-1) generates real data at IF instead
of complex baseband signal. Previously this is handled by the fallback
downconvert! function, but that is extremely slow because it doesn't use
SIMD.

Vector type is not compatible with SubArray type, use AbstractVector
instead