[wip] Legendre triples
some modifications in EGSZ2 marking
Merge branch 'sgfem' of https://github.com/SpuqTeam/spuq into sgfem

Conflicts:
	src/spuq/math_utils/multiindex.py
	src/spuq/sgfem/legendre_triple.py
	src/spuq/sgfem/tensor_operator.py
	src/spuq/sgfem/test/test_hermite_triple.py
	src/spuq/sgfem/test/test_tensor_operator.py