optimized bloch_redfield_tensor routine
optimized bloch_redfield_tensor routine

Mostly by replacing explicit loops with vectorized numpy expressions.
For a test simple Hamiltonian with a single N-level bosonic operator,
it is faster by a factor of ~40 for N=100 (Hamiltonian 100x100),
i.e., it takes 12 seconds instead of 460 seconds. The speedup improves
about linearly with N for this system.

zasdfgbnm/qutip,zasdfgbnm/qutip,anubhavvardhan/qutip,cgranade/qutip,qutip/qutip,qutip/qutip,cgranade/qutip,anubhavvardhan/qutip