ADD: `radar_freq` param in `hydroclass_semisupervised`

[RBhupi]

• [x] Documentation reflects changes

1. Added radar_freq option when frequency information is missing from the radar object.
2. Now always print warning showing frequency used by the function.
3. Added docstring for thechange.
4. Modified the old notes section to include radar_freq.

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[RBhupi]

This is failing because of the missing file. I want to use this file in my example which I can take from the ARM servers.

WARNING: ../../examples/retrieve/plot_hydrometeor_class_x_band.py unexpectedly failed to execute correctly:

Traceback (most recent call last):
  File "/home/runner/work/pyart/pyart/examples/retrieve/plot_hydrometeor_class_x_band.py", line 29, in <module>
    radar = pyart.io.read(files[0])
                          ~~~~~^^^
IndexError: list index out of range

[zssherman]

@RBhupi If you want to pull the file and run that, you can do a PR adding the file to https://github.com/openradar/open-radar-data

then once the file has been added to the data directory, run python make_registry.py

[zssherman]

@RBhupi This looks good to me now!