Adds slice method and associated tests

[MohamedAli1937]

PR Description

This PR adds the slice() method to GenericSpectrogram to make it easier to extract a subset of the spectrogram by time and/or frequency. Previously, users had to manually compute array indices for slicing, which was error-prone.

fixes: #166

With this method:

• spec.slice(time=(t_start, t_end), freq=(f_min, f_max)) returns a new GenericSpectrogram containing only the requested subset.

• Metadata (times, freqs, start_time, end_time, wavelength) is automatically updated.

• Original spectrogram remains unchanged.
  Example:

# Before slice method
sliced_data = data[1:5, 1:4]
sliced_times = times[1:5]
sliced_freqs = freqs[1:4]

# After slice method
sliced_spec = spec.slice(time=(times[1], times[4]), freq=(freqs[1], freqs[3]))
print(sliced_spec.times)       # [t1, t2, t3, t4]
print(sliced_spec.frequencies) # [f1, f2, f3]

AI Assistance Disclosure

AI tools were used for:

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding
• No AI tools were used

Regardless of AI use, the human contributor remains fully responsible for correctness, design choices, licensing compatibility, and long-term maintainability.

Screenshots:

test passed:

[samaloney]

Hey thanks for the PR the implementation looks solid. But please see my comment on the corresponding issue.