Fix: Python to MATLAB string array conversion

Conversion from a py.list into a `string` array fails in MATLAB versions
older than 2022a.

Author: simmsa

mhkit/utils/convert_numeric_dataframe_to_struct.m

@@ -17,11 +17,11 @@
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
-    columns = string(py.list(df.columns.values));
+    columns = cell(df.columns.values.tolist());
     result_struct = struct();
 
     for i = 1:length(columns)
-        this_column = columns{i};
+        this_column = char(columns{i});
         result_struct.(this_column) = double(df.get(this_column).values.tolist());
     end
 end