Fixes error pointed out by Jinliang in SOC interpolation function (#1314

)

* Fixes error pointed out by Jinliang in interpolation function

* add note

* fix typo

---------

Co-authored-by: Ludovico Bianchi <lbianchi@lbl.gov>

idaes/models_extra/power_generation/unit_models/soc_submodels/common.py

@@ -182,11 +182,12 @@ def _interpolate_2D(
         phi_bound_1: expression for the value of the quantity to be interpolated
             at the 1 bound
         derivative: If True estimate derivative
-        method: interpolation method currently only CDS is supported
 
     Returns:
         expression for phi at face
     """
+    # TODO add tests to ensure this function works as designed
+    # Also, user beware, use of CV_Bound enums is untested
     icu = ic - 1
     icd = ic
     if ic == ifaces.first():
@@ -203,7 +204,7 @@ def _interpolate_2D(
     if not derivative:
         cf = faces.at(ic)
         lambf = (cd - cf) / (cd - cu)
-        return (1 - lambf) * phi_func(icu) + lambf * phi_func(icd)
+        return (1 - lambf) * phi_func(icd) + lambf * phi_func(icu)
     else:
         # Since we are doing linear interpolation derivative is the slope
         # between node centers even if they are not evenly spaced