[python] Various fixes regarding function space basis trees
This MR contains several fixes that bring the Python interface closer to the C++ interface.
Some of them are not backward-compatible. I have decided to not care, because I don't think anybody is already seriously using this interface.
Also, I have deliberately not written the changes to CHANGELOG.md
. Instead, I plan to write a generic "The Python interface has seen lots of improvements" message before the next release.