Do not accept fplan files for simulations without an orbit files

Frequency plans are computed for given orbits (given in the form of an orbit file), and should only be used in simulations using the same orbit file.

Currently LISA Instrument allows the user to set a custom set of LTT series (more exactly PPRs) through the orbit parameter (or choose a static set of PPRs, or give the path to an orbit file). The user can also, independently, pass the path to an fplan file through the fplan parameter (or choose a default static set of beatnote frequencies, or give a custom set of such beatnotes). This means that the user can, by accident, pick a fplan file and use custom PPRs (not from an orbit file), which is surely wrong.

We should not allow this, and issue an error. This matches the behavior of LISANode, see https://gitlab.in2p3.fr/j2b.bayle/LISANode/-/issues/385.