Resolve "Do not accept fplan files for simulations without an orbit files"

Closes #138 (closed)

• Raise an error when the path to a frequency-plan file is given with custom PPRs (ie., when no orbit files are given)

lisainstrument/instrument.py

@@ -537,6 +537,9 @@ class Instrument:
         elif isinstance(fplan, str):
             logger.info("Using frequency-plan file '%s'", fplan)
             self.fplan_file = fplan
+            # Refuse to use a fplan file if no orbit files are used
+            if self.orbit_file is None:
+                raise ValueError("cannot use frequency-plan for non orbit files")
             # Without a standard lock config, there is no dataset
             # in the frequency-plan file and therefore we cannot use it
             if self.lock_config is None: