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

@@ -539,8 +539,13 @@ class Instrument:
         Args:
             fplan: `fplan` parameter, c.f. `__init__()`
         """
+        # pylint: disable=too-many-branches
         if fplan == 'static':
-            logger.info("Using default set of locking beatnote frequencies")
+            logger.warning(
+                "Using default set of locking beatnote frequencies; this "
+                "might cause interferometric beatnote frequencies to fall "
+                "outside the requirement range of 5..25 MHz"
+            )
             self.fplan_file = None
             self.fplan = ForEachMOSA({
                 '12': 8E6, '23': 9E6, '31': 10E6,
@@ -549,6 +554,9 @@ 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: