on entry: jd,fract have the desired date/time, or, if jd=0, then
just initialize
on exit:  Ihoc=0 indicates there is a correction in HOCCOM common
Ihoc=1 indicates no correction

line 2
Jd1hoc - ephemeris start epoch (pep convention)           i7
Jd2hoc - ephemeris end epoch (pep convention)             i8
the ephemeris must include a block covering the end of day Jd2hoc,
even if the correction is zero

subsequent lines
Thoca - time block start epoch (noon convention)          f10.2
Thocb - time block end epoch (noon convention)            f11.2
Hocx  - ad hoc correction to longitude libration (nrad)   f8.2, 8x
Hocy  - ad hoc correction to latitude libration (nrad)    f8.2