Packaging for the v1.2.1 Release

size_t is unsigned but not Eigen::Dynamics (which is set to -1)
This set Dim to the maximal value for a long unsigned int.
This generates overflow.
This commit fix this problem.

[CI] Add Coverage & Badges

Fix python3, update tests, CI & generate doc

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)