Adding bmi_conversion.py

[Snimer]

Describe your change:

Adding bmi_conversion.py
It converts Mass and Height into Body Mass Index.
It contains two functions. One to convert BMI from Metric units and one from US units.

• 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 have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[l3str4nge]

Please add following doctest to us_units_bmi:

>>> us_units_bmi(130, 0)
  WHAT RESULT WE SHOULD EXPECT?

[Snimer]

Please add following doctest to us_units_bmi:

>>> us_units_bmi(130, 0)
  WHAT RESULT WE SHOULD EXPECT?

@mateuszz0000
I am closing this pull request because I deleted the branch from where the file was.
So, I am made a different pull request. Please review this one-
#3727