Convert an epoch to Besselian or Julian to match another one.
D = sla_EPCO (K0, K, E)
form of result: `B'=Besselian, `J'=Julian
form of given epoch: `B' or `J'
the given epoch converted as necessary
The result is always either equal to or very close to
the given epoch E. The routine is required only in
applications where punctilious treatment of heterogeneous
mixtures of star positions is necessary.
K0 and K are not validated. They are interpreted as follows:
If K0 and K are the same, the result is E.
If K0 is `B' and K isn't, the conversion is J to B.