r.b. goldstein  r.king  march 1978
routine to calculate any or all of the following:
r : magnitude of input vector
u : unit vector
du: derivative of unit vector (wrt time)
the input vector, x, contains position and velocity

ictl: binary coded control integer
1 bit=1:  calculate r
2 bit=1:  calculate u
4 bit=1:  calculate du

if any of the bits are not set, and the associated quantity
is needed further on in the routine (i.e. r is needed
to calculate u) it is assumed that
the correct quantities were sent throuth the calling
statement.