Write `create_test_gradient` and `is_gradient`

[richelbilderbeek]

Needed for #13

Tests:

assert is_gradient.__doc__
assert is_gradient("nonsense") == False
assert is_gradient(create_test_gradient()) == True

[emiliamoralest]

completed in is_gradient.py

[richelbilderbeek]

I've seen the code in learners/src/is_gradient.py. Well done!

However, this code is not part of the project yet, hence it cannot be used by others yet. To solve this:

• Put the function is_gradient in file src/bacsim/gradient.py
• Put the tests in file tests/test_gradient.py

Also, the function create_test_gradient is undefined. Please create and test it. Here are some minimal tests:

assert create_test_gradient.__doc__

Because this function is missing, the test below will fail:

assert is_gradient(create_test_gradient()) == True

Please check if your tests pass next time, especially before pushing to the main branch :+1:

Good luck!