make arithmetic expression case insensitive

internally, we use downcased names

slurm-eff-tool.py

@@ -211,10 +211,12 @@ _ALLOWED_CMPOPS = {
 ####################################
 class SafeExpression:
     def __init__(self, expression: str):
-        self.expression = expression
-        self.tree = ast.parse(expression, mode="eval")
+        self.expression = expression.lower()
+        self.tree = ast.parse(self.expression, mode="eval")
 
     def evaluate(self, variables: Mapping[str, int | float]) -> int | float | bool:
+        # normalize to downcased variable names
+        variables = {k.lower(): v for k,v in variables.items()}
         return self._eval(self.tree.body, variables)
 
     def _eval(self, node: ast.AST, variables: Mapping[str, int | float]):