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Fix Cython build dependency with pyproject.toml

Fix Windows MSVC install by updating Eigen Library

Maybe also Fixes #60? The function isn't called anywhere and modern msvc
do define it already, so the definition leads to an error.

Fixes #49 #52

The file `test.py` is now moved into the test folder and pytest directives have been added.

Now it explicitly checks for there to be an all-0 pixel meaning
"unknown" and prints an info about it if it finds one.
This is a try at fixing #30 and #31.

The exception is now correctly propagated instead.

add grayscale image example

Specifically, about what the compatibility and kernel parameters are, about missing the weights, and some pointers about learning.