Style fix for LookupTable class

A method in OptimizationProblem was returning a "LookupTable" type, with
the class not defined anywhere in scope. One of the CI/CD style checkers
became a bit more aggressive recently and started to catch this as an
error.

We now properly defined a base LookupTable class, from which the one in
csv_lookup_table_mixin.py inherits.

src/rtctools/optimization/csv_lookup_table_mixin.py

@@ -18,12 +18,13 @@
 from scipy.interpolate import bisplev, bisplrep, splev
 from scipy.optimize import brentq
 
+from .optimization_problem import LookupTable as LookupTableBase
 from .optimization_problem import OptimizationProblem
 
 logger = logging.getLogger("rtctools")
 
 
-class LookupTable:
+class LookupTable(LookupTableBase):
     """
     Lookup table.
     """

src/rtctools/optimization/optimization_problem.py

@@ -19,6 +19,26 @@
 BT = Union[float, np.ndarray, Timeseries]
 
 
+class LookupTable(metaclass=ABCMeta):
+    """
+    Base class for LookupTables.
+    """
+
+    @property
+    def inputs(self) -> List[ca.MX]:
+        """
+        List of lookup table input variables.
+        """
+        raise NotImplementedError
+
+    @property
+    def function(self) -> ca.Function:
+        """
+        Lookup table CasADi :class:`Function`.
+        """
+        raise NotImplementedError
+
+
 class OptimizationProblem(DataStoreAccessor, metaclass=ABCMeta):
     """
     Base class for all optimization problems.
@@ -389,7 +409,7 @@ def constant_inputs(self, ensemble_member: int) -> AliasDict[str, Timeseries]:
         """
         return AliasDict(self.alias_relation)
 
-    def lookup_tables(self, ensemble_member: int) -> AliasDict[str, 'LookupTable']:
+    def lookup_tables(self, ensemble_member: int) -> AliasDict[str, LookupTable]:
         """
         Returns a dictionary of lookup tables.