Blend-modes

Changed
- Removed tests of Python versions < 3.8

Fixed
- Fixed outdated comparison of Numpy nans
- Fixed tests that used deprecated np.float

Added
- Added images in blending function documentation

Changed
- Dropped support for Python versions < 3.5
- Refactored internal package structure, splitting type checks and image blending functions into separate modules
- Improved error messages for type checks to be more user-friendly
- Improved visual example gallery in documentation
- Ported tests from unittest framework to pytest framework
- Updated links in readme file

Fixed
- Image blending functions no longer modify their inputs

Fixed
- Fixed bug where it would not be possible to import the package

Fixed
- Bug fix in setup.py for package creation

Added
- Added changelog

Changed
- Changed overlay mode to deliver Adobe-style results. Use soft light mode for backwards compatibility.
- Changed docstrings to Google style.
- Improved API to make blending methods more accessible