I/Q replay without mmap() usage

On a host with less than 4GB RAM, call to mmap() to replay big IQ file might fail because the RAM is too small. To allow replaying IQ on such a host, IQ shall be loaded on a per subframe basis. We need to consider also per subframe writing on disk for record mode. Choice between mmap() or individual subframe read/write from file could be done automatically depending on available RAM at runtime.