Allow python3 thanks to newer versions of CMake submodule

[python/spline] create spline by concatenating splines.

[polynomial] fix order=0 case. this makes constant splines possible

Change headers to UTF8 and fix problem with >>

Fix python/CMakeLists.txt (add a custom target to make sure that the proper python file is generated)

Minor fix. Remove warnings.

It was detected thanks to the tests put back in business.
The results are not consistent but at least people can work with them.