Block Lower Triangular Transform

Mentioned in https://staff.polito.it/roberto.zanino/sub1/teach_files/modelica_minicourse/03%20-%20Symbolic%20Manipulation.pdf . It can make computations a lot faster on DAEs. Mentioned in https://uwspace.uwaterloo.ca/bitstream/handle/10012/5844/Uchida_Thomas.pdf?sequence=1 using a Groebner basis method.