Add option to Huffman Code a string not just a file; add doctests to Huffman file

[Lecdi]

Describe your change:

Previously the compression/huffman.py file provided the huffman function to Huffman Code the contents of a file and print the result.

I have added the huffman_string function (along with parse_string, which is analogous to parse_file) to Huffman Code a given string and return the result.

I have also edited the huffman function to allow the user to specify more options, such as how letter bitstrings should be separated and which file to print the result to. Calls to the huffman function with only one argument still do exactly the same thing as before.

I have also added doctests to the existing build_tree function.
EDIT: no longer doctests for existing huffman and parse_file functions

• [✔ ] Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• [✔ ] Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?
• [ ]
  (I don't know if huffman_string counts as a new algorithm, I suppose it does)
  (I see it says avoid changing both code and tests in a single pull request, but since I was adding my own new functions with doctests, I thought it made sense to also add doctests for the existing functions)

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".

[Lecdi]

It can't find the files compression/text_data/text_original.txt or compression/text_data/text_huffman.txt. I have tried with and without the compression prefix and neither works. Does anyone know what the root directory is for opening files or what path I should give?

All of the doctests are passing on my home computer...

[Lecdi]

OK all the tests are passing now but there are no doctests on parse_file and huffman. I couldn't get opening files in a doctest to work. Maybe someone else can add those doctests in the future.

There are still doctests on the new functions parse_string and huffman_string which were the main point of the pr anyway.

There are still new doctests on existing function build_tree which didn't have any previously.