Added copyto!(::DensityMatrix)

[jlbosse]

Same motivation as adding similar(::DensityMatrix): It is desirable to get as many features of ArrayReg as possible to work for DensityMatrix as possible, so that most code written with a ArrayRegister in mind also works with DensityMatrix.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.08 :tada:

Comparison is base (dc9d152) 88.24% compared to head (c3738bb) 88.32%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #460 +/- ## ========================================== + Coverage 88.24% 88.32% +0.08% ========================================== Files 77 76 -1 Lines 4807 4806 -1 ========================================== + Hits 4242 4245 +3 + Misses 565 561 -4 ``` | [Impacted Files](https://codecov.io/gh/QuantumBFS/Yao.jl/pull/460?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=QuantumBFS) | Coverage Δ | | |---|---|---| | [lib/YaoArrayRegister/src/density\_matrix.jl](https://codecov.io/gh/QuantumBFS/Yao.jl/pull/460?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=QuantumBFS#diff-bGliL1lhb0FycmF5UmVnaXN0ZXIvc3JjL2RlbnNpdHlfbWF0cml4Lmps) | `96.55% <100.00%> (+0.12%)` | :arrow_up: | | [lib/YaoBlocks/src/channel/error\_channel.jl](https://codecov.io/gh/QuantumBFS/Yao.jl/pull/460?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=QuantumBFS#diff-bGliL1lhb0Jsb2Nrcy9zcmMvY2hhbm5lbC9lcnJvcl9jaGFubmVsLmps) | `100.00% <100.00%> (ø)` | | | [lib/YaoAPI/src/interface.jl](https://codecov.io/gh/QuantumBFS/Yao.jl/pull/460?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=QuantumBFS#diff-bGliL1lhb0FQSS9zcmMvaW50ZXJmYWNlLmps) | | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=QuantumBFS). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=QuantumBFS)

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[jlbosse]

The CI failure seems to have something to do with julia servers being down, not with this PR

[Roger-luo]

LGTM