Fix plot weather data func

[chenzhao299]

Addressed various fixes from Marco and Chad's reviews:

• Marco:

  • Simplified the repeated codes in plot_weather_data() function
  • Set default parameters for time_basis and col_name argument of plot_weather_data() function
  • Fix single quote mark problem in plot_weather_data() function's docstring
  • Add explaination of how to get the obs_df in plot_weather_data() function's docstring

• Chad:

  • Delete tests/test_noaastn.py file
  • Written plot_weather_data() function in a more DRY fashion based on the suggestion, thank you for that!
  • Labled x-axis nicely when using the plot_weather_data() function with "daily"argument

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