Fix error in setting an element of a pq grid

Fix error in setting an element of a pq grid

Fixed error in setting an element of a pq grid; it was trying to set
the element of the magnitude to the pq scalar, now setting the element
to the magnitude of the scalar.  Now works:
ANTIK-USER(3): (defparameter test #_#m(1.0d0 2.0d0 3.0d0)_km)
TEST
ANTIK-USER(4): test
#_#(1000.000000000000000d0 2000.000000000000000d0 3000.000000000000000d0)_m
ANTIK-USER(5): (setf (grid:aref test 0) #_13.0d0_km)
#_13000.000000000000000d0_m
ANTIK-USER(6): test
#_#(13000.000000000000000d0 2000.000000000000000d0 3000.000000000000000d0)_m