Proposal: Add Generalized Gell-Mann matrices for n-qudits

I think that a function to generate parameterized GGM matrices will be useful especially on TFQ for the PQCs (they are used in [Quantum Convolutional Neural Networks (arXiv:1810.03787v2)](https://arxiv.org/abs/1810.03787.pdf) for example). 

More info about GGMs in [Bloch vectors for qudits (arXiv:0806.1174v1)](https://arxiv.org/abs/0806.1174.pdf)