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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.

There was a sudden interest in this, both via issues (#23) and direct e-mail almost simultaneously. Odd 😄

Clearer and more unified unary utilities.