Add utility functions + minor improvements

[msamsami]

• Add a script for utility functions containing the following two functions:
  • is_dist_supported
  • get_dist_class
• Use the above utility functions in the GeneralNB implementation
• Convert to NumPy array even if priors is of type set or tuple in GeneralNB and GaussianWNB
• Set the minimum version of pytest package to 7.0.0
• Include the index of the distribution item (in distributions) in the error message when the distribution is not supported
• Minor improvements