Add concept indirectly_copyable to constrain the DenseMatrixAssigner

Summary

Add concept definition Std::indirectly_copyable that checks whether the value provided by one iterator can be assigned to the reference provided by a second iterator. This concept is then used to constrain the DenseMatrixAssigner properly. The old constraint was not precise enough and lead to some unintended constructor invocations.

added cleanup concepts labels

requested review from @santiago.ospina

assigned to @simon.praetorius

added 3 commits

• 93987435 - Add missing concept indirectly_copyable
• 78777472 - Constrain the DenseMatrix assigner
• 6afd2a2e - Add changelog entry

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mentioned in merge request !1401

approved this merge request

Note that the indirectly_copyable concept is lacking in libc++-13, which is in our current support-list. It is supported with libc++-15.

added 1 commit

• 3476af35 - Remove check for .N() and .M() in DenseMatrix assigner doe to incomplete type problems

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added 1 commit

• 0fcefe96 - Remove check for existence of concepts header

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resolved all threads

merged

mentioned in commit c8801253