Calculate the suncenter position (in SXT pixel coordinates) from
the HXA info in the ATR (or PNT) files. Tries to reconstruct hidden limbs.
IDL> sunc = hxa_suncenter(atr)
IDL> sunc = hxa_suncenter(index=index)
If the input atr or index have N elements, then the
result will be a floating array of 4xN elements. sunc(0,*)
is the SXT column number in `IDL coordinates'
, sunc(1,*)
is the SXT line number, sunc(2,*)
is the milliseconds of day for the input time, and sunc(3,*)
is the days since 1-Jan-79 number for the input time.