chore: get_rolling_fn must accept CAGR as well as cagr parameter values

Author: chilango74

okama/common/helpers/helpers.py

@@ -224,9 +224,9 @@ def get_annual_return_ts_from_monthly(
         """
         Annual Rate of Returns time series from monthly data.
         """
-        if return_type == "cagr":
+        if return_type.lower() == "cagr":
             ts = ror_monthly.resample("A").apply(lambda x: np.prod(x + 1.0) - 1)
-        elif return_type == "arithmetic_mean":
+        elif return_type.lower() == "arithmetic_mean":
             ts = ror_monthly.resample("A").sum()
         else:
             raise ValueError("Return type must be either cagr or arithmetic_mean.")