Chaining en passant is inconsistent with the manual

[kreibaum]

As @yagu0 noted, chaining en passant is inconsistent with the official game rules as defined in the manual.

After the capture, a released piece starts at the capturing square on the interface, but at the initial end square in the PDF.

So this is a detail where Felix and I need to decide which version is correct.

[kreibaum]

While this is marked as "bug", I like my version better. I think once variants are done with #57 then this can be added.