More reorg and refactoring of the code.

More reorg and refactoring of the code.

Added a convenience function to estimate the time to intercept the selected
radial give the current heading and speed.  This can be useful to a flight
directory to compute the point to switch from armed to coupled mode at just
the right time so the pilot can roll out onto the desired heading on the
desired radial.

Add a first whack at estimating a ground track heading error (difference
between aircraft heading and ground track directon.)  This needs more work
and testing.