Usual vs. Unusual Values

Come up with two functions, one that takes in a list and returns a list of usual values and another that returns a list of unusual values.

Look at existing functions in the repository to understand the package's style and conventions. Determine what 'usual' and 'unusual' values mean in a statistical context—likely based on measures like mean and standard deviation. Implement two functions, probably in a new or existing module, and ensure they handle lists appropriately. Write tests to verify the behavior.

Written by the indexing model from the issue text.