No move semantics

Hello!

I think it would be great if multi_array had move semantics, at the moment unfortunately it is inefficient to return multi_arrays by value and the workaround is not great:

https://stackoverflow.com/questions/42698451/how-to-stdmove-a-boostmulti-array#comment72520077_42698451

I think having move construction / assignment would be great

The issue requests move construction and assignment for multi_array but names no files or tests. First locate the multi_array implementation and its test suite, then define completion as supported move construction and assignment with regression coverage.

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