Decouple pattern from simulator backends

[masa10-f]

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Currently, each simulator backend depends on the pattern class, but they, in principle, only require a part of the information from the pattern. It would be beneficial if each backend were separated from the pattern and became self-contained.

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Remove the dependency on the pattern class.

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The issue related to Pattern._pauli_preprocessed(#168 ) should be fixed before this issue
#167 can be easily done after this issue.

[mgarnier59]

Have a look at #178. It is done there.

[masa10-f]

Thanks! I'll look it over later