v0.8.8

Yao v0.8.8

Diff since v0.8.7

Merged pull requests:

• Added docstring to pauli_error_channel() (#459) (@jlbosse)
• Added copyto!(::DensityMatrix) (#460) (@jlbosse)
• More density matrix support (#461) (@jlbosse)
• fix: correct citation based on Qiang Miao's comment (#463) (@Roger-luo)