The waterfall file has now a constant sized header of 52 bytes,
so that plotting tools can reconstruct properly the spectrum.

The structure of the header is the following:
  - A 32 byte string containing the timestamp in
    ISO-8601 format. This timer has microsecond accuracy.
  - A 4 byte integer containing the sampling rate
  - A 4 byte integer with the FFT size
  - A 4 byte integer containing the number of FFT snapshots for one row
    at the waterfall
  - A 4 byte float with the center frequency of the observation.
  - A 4 byte integer indicating the endianness of the rest of the file. If
    set to 0 the file continues in Big endian. Otherwise, in little endian.
    The change of the endianness is performed to reduce the overhead at the
    station.

 Note that all contents of the header are in Network Byte order! The rest
 of the file is in native byte order, mainly for performance reasons.
 Users can use data of the header to determine if their architecture match
 the architecture of the host generated the waterfall file and act
 accordingly.

 The file continues with information regarding the spectral content of the
 observation.
 Each waterfall line is prepended with a int64_t field indicating the
 absolute time in microseconds with respect to the start of the waterfall
 data (stored in the corresponding header field).
 The spectral content is stored in $FFT$ float values already converted in
 dB scale.