Avoid unnecessary time frequency resampling

[Hem-W]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Unexpected time resampling leads to extremely large dask graphs in default installation #2111
• Tests for the changes have been added (for bug fixes / features)
  • (If applicable) Documentation has been added / updated (for bug fixes / features)
• CHANGELOG.rst has been updated (with summary of main changes)
  • Link to issue (:issue:number) and pull request (:pull:number) has been added

What kind of change does this PR introduce?

• Avoid unnecessary time resampling in preprocess_standardized_index when freq is not None but the same as the input data.

Does this PR introduce a breaking change?

No

Other information:

I am thinking whether it is necessary to prompt users to install flox when they input dask datasets and require time resampling.

[github-actions]

Warning

This Pull Request is coming from a fork and must be manually tagged approved in order to perform additional testing.

[coxipi]

Hi @Hem-W !

Thanks for your insight here and in your issue. That's a great point. I think constraining the fast implementation to freq=None is counter-intuitive. If you give a daily input dataset and specify freq="D", you would be right to expect that resampling should be avoided.

@aulemahal @Zeitsperre , have your thoughts evolved on the inclusion of flox by default or not?

[aulemahal]

I think this works!

[Zeitsperre]

@coxipi Outside the CI issues (being addressed elsewhere), it looks like there's one actual failing test:

  =================================== FAILURES ===================================
  _ TestStandardizedIndices.test_standardized_streamflow_index[MS-1-fisk-ML-values12-0.02] _
...
   tests/test_indices.py:982: 
  _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
  
  args = (<function assert_allclose.<locals>.compare at 0x13cc64400>, array([ 0.32619351, -1.57770013, -0.43633094,  0.25251413, -0.81498785]), array([ 0.533154, -1.5777  , -0.436331,  0.29581 , -0.814988]))
  kwds = {'equal_nan': True, 'err_msg': '', 'header': 'Not equal to tolerance rtol=0, atol=0.02', 'strict': False, ...}
  
      @wraps(func)
      def inner(*args, **kwds):
          with self._recreate_cm():
  >           return func(*args, **kwds)
  E           AssertionError: 
  E           Not equal to tolerance rtol=0, atol=0.02
  E           
  E           Mismatched elements: 2 / 5 (40%)
  E           Max absolute difference among violations: 0.20696049
  E           Max relative difference among violations: 0.38818145
  E            ACTUAL: array([ 0.326194, -1.5777  , -0.436331,  0.252514, -0.814988])
  E            DESIRED: array([ 0.533154, -1.5777  , -0.436331,  0.29581 , -0.814988])
  
  /Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/contextlib.py:81: AssertionError

Once the CI has been fixed here, could you quickly verify that this is expected?

[Zeitsperre]

@Hem-W Thanks so much for the code contribution!