Replace pytweening with local easing functions

[flpm]

Fixes #157

What was done:

• Created new easing functions in plotting/animation
• Replaced pytweening easing functions in morpher.py with the new functions
• Removed pytweening as a dependency from pyproject.toml
• Replaced 'python' with sys.executable in CLI test

[stefmolin]

Thanks for the PR, @flpm!

Let's also include individual tests for each of the new functions. I know they are tested indirectly as part of the DataMorpher, but it would be helpful to have pure tests of their functionality for future changes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.42%. Comparing base (2b0666e) to head (c79215d).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #158      +/-   ##
==========================================
+ Coverage   98.38%   98.42%   +0.03%     
==========================================
  Files          43       43              
  Lines        1737     1775      +38     
  Branches      345      358      +13     
==========================================
+ Hits         1709     1747      +38     
  Misses         25       25              
  Partials        3        3              

Files	Coverage Δ
src/data_morph/morpher.py	100.00% <100.00%> (ø)
src/data_morph/plotting/animation.py	97.61% <100.00%> (+4.28%)	⬆️
tests/plotting/test_animation.py	100.00% <100.00%> (ø)
tests/test_main.py	100.00% <100.00%> (ø)