[python] Remove non-standard shortcut for vector-valued Lagrange bases
The Python interface had a shortcut that allowed to create vector-valued Lagrange bases by an optional 'dimRange' parameter of the Lagrange basis constructor. The shortcut was merely syntactic sugar: Internally a power basis was used.
Since the C++ interface doesn't have this extra 'dimRange' parameter, and since no other bases in Python has it either, this commit removes the 'dimRange' parameter for the sake of consistency.