Incorrect benchmark for dense operator addition in Python

[akhmerov]

Contrary to the description, the file below tests the timing of addition of two sparse matrices and not dense ones.

https://github.com/qojulia/QuantumOptics.jl-benchmarks/blob/master/benchmarks-QuTiP/addition_dense_dense.py

[akhmerov]

Same applies to dense multiplication—the Python version isn't dense.

[david-pl]

That is quite the copy-paste error you found there, thanks. I changed the code where necessary.