Add a method to get _ts.tv_nsec (nanoseconds) output

[Engineero]

minmea appears to read milliseconds from the GPS (stored as nanoseconds in _ts.tv_nsec), but the getTime method only outputs full seconds (_ts.tv_sec). It's not clear to me whether the GPS chip itself actually registers milliseconds, but either adding the nanoseconds to getTime or adding a new method to return just the nanoseconds value would help get more precise timing information to downstream applications.

I've messed with this on my own, but my CPP is weak and I don't think I'm compiling the library right to use my updates. I've added the following to GPS.cpp with signature in GPS.h.

unsigned long GPSClass::getTimeNs()
{
  return _ts.tv_nsec;
}

I think some discussion is merited on whether it makes sense to do it this way or add something to return a floating-point time value from getTime, something like:

unsigned long GPSClass::getTime()
{
  return _ts.tv_sec + _ts.tv_nsec * 1e-9;
}