Remove duplicate implementation of Collatz sequence

[tianyizheng02]

Describe your change:

Fixes#8061

• Removed maths/3n_plus_1.py in favor of maths/collatz_sequence.py because the latter is the more well-known name for the problem
• Added tests from maths/3n_plus_1.py to maths/collatz_sequence.py
• Converted the collatz_sequence function to a generator to save memory
• Expanded explanation of the Collatz conjecture

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[cclauss]

Looking cool! Do these changes effect performance?

[tianyizheng02]

Looking cool! Do these changes effect performance?

@cclauss I don't think performance is really affected in most cases since the algorithm itself is basically unchanged. However, I think there'd be slightly better performance if one were to iterate over the algorithm's output:

fornumincollatz_sequence(n): # do something with num

since having a generator means not needing to create and store the entire sequence in memory. However, I haven't timed this generator implementation against the original list implementation, so I don't know how much of a difference it makes.

[cclauss]

Nice!