Fix: enforce consistency between solvation method and solvent

Raise a ValueError when only one of solvation_method or solvent is defined.
Both must be provided together or both must be None.

Author: LeenFahoum

arc/job/adapters/xtb_test.py

@@ -96,7 +96,7 @@ def setUpClass(cls):
         cls.job_9 = xTBAdapter(execution_type='queue',
                                job_type='opt',
                                project='test_9',
-                               level=Level(method='xtb', solvent='water'),
+                               level=Level(method='xtb', solvent='water', solvation_method="alpb"),
                                project_directory=os.path.join(ARC_PATH, 'arc', 'testing', 'test_xTBAdapter_9'),
                                species=[ARCSpecies(label='spc2', smiles='CC[O]')],
                                )

arc/level.py

@@ -83,10 +83,11 @@ def __init__(self,
                      or solvation_scheme_level.solvation_scheme_level is not None):
             raise ValueError('Cannot represent a solvation_scheme_level which itself has solvation attributes.')
         self.solvation_scheme_level = solvation_scheme_level
-        if self.solvation_method is not None and self.solvent is None:
-            raise ValueError(f'Cannot represent a level of theory with a solvation method ("{self.solvation_method}") '
-                             f'that lacks a solvent.')
-
+        if (self.solvation_method is None) != (self.solvent is None):
+            raise ValueError(
+                'Both solvation method and solvent must be defined together, or both must be None. '
+                f'Got solvation method = "{self.solvation_method}", solvent = "{self.solvent}".'
+            )
         self.args = args or {'keyword': dict(), 'block': dict()}
 
         if self.repr is not None: