Select epoch prefix 'B' or 'J'.
jb int slaDbjin prefix status: 0=none, 1='B', 2='J'
e double epoch - Besselian or Julian
*k char 'B' or 'J'
*j int status: 0=OK
If jb=0, B is assumed for e < 1984.0, otherwise J.
P.T.Wallace Starlink 23 October 1993