determine a1-ut1 time from 1956 to the present.  the source is
controlled by jct(33):
jct(33)=-1  use usno values tabulated in this routine
= 0  (default) after 3 jan 1969 use bih values
tabulated in subroutine bihut1;  before 3 jan
1969 use usno values
.gt.0 read A.1-UT1 and wobble tables from data sets
JCT(33) and JCT(33)+1, respectively.