The units of PV.
This integer is a sum of some unit constants (CALCEPH_UNIT_???) and/or the constant USE_NAIFID.
If the unit contains USE_NAIFID, the NAIF identification numbering system is used for the target (NAIF identification numbers for the list).
If the unit does not contain USE_NAIFID, the old number system is used for the target (see the list in the function calcephpy.CalcephBin.compute()).