Continuing Fix: Python to MATLAB string array conversion

This code attempts to create a MATLAB double from a numpy array

MATLAB versions older that 2022a are throwing this error:

```
Error ID:
      ---------
      'MATLAB:invalidConversion'
      --------------
      Error Details:
      --------------
      Error using double
      Conversion to double from py.list is not possible.
```

mhkit/utils/convert_numeric_dataframe_to_struct.m

@@ -1,27 +1,31 @@
 function result_struct = convert_numeric_dataframe_to_struct(df)
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-% Returns
+%
+% Converts a numeric pandas DataFrame to a MATLAB struct.
 %
 % Parameters
 % ------------
-%    delft_3d_py_object: py.netCDF4._netCDF4.Dataset
-%       A netCDF python object.
-%    seconds_run: number
-%       Time index
+%    df: DataFrame
+%       A numeric pandas DataFrame.
 %
 % Returns
 % ---------
-%    result: number
-%        The closest time index
+%    result_struct: struct
+%        A MATLAB struct containing the data from the DataFrame.
 %
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
+   % Guard to check type of df
+   if ~isa(df, 'py.pandas.core.frame.DataFrame')
+       error('MATLAB:convert_numeric_dataframe_to_struct:InvalidInput', 'df must be a pandas DataFrame.');
+   end
+
     columns = cell(df.columns.values.tolist());
     result_struct = struct();
 
     for i = 1:length(columns)
         this_column = char(columns{i});
-        result_struct.(this_column) = double(df.get(this_column).values.tolist());
+        result_struct.(this_column) = double(py.numpy.array(df.get(this_column).values));
     end
 end