[python] Use the same default index merging strategies as C++

Using composite or power nodes in C++ yields blocked multi-indices by default. So far, however, the Python interface used flat indices by default. This violates the Principle of Least Surprise, and there this commit changes the Python defaults to match those of the C++ interface.

To make the tests pass even with the new defaults, this MR also implements the size methods of DefaultGlobalBasis.