This routine sets a new value for the UNITS component of an
existing NDF data structure. The NDF is accessed in update mode
and any pre-existing UNITS component is over-written with a new
value. Alternatively, if a `null' value (!) is given for the
UNITS parameter, then the NDF's UNITS component will be erased.
There is also an option to modify the pixel values within the NDF
to reflect the change in units (see Parameter MODIFY).