2d tensor projectors v2

Added 2d tensor product projectors which take as input two projectors_global_1d objects. A unit test for the new 2d projectors has been added in tests_projectors_global.py. The docstrings for projectors_global_1d have been updated. The methods "rhs_0" and "rhs_1" in projectors_global_1d have been renamed to "dofs_0" and "dofs_1", respectively.