Remove xtensor

[jorgensd]

Remove xtensor as md array dependency in C++ layer (following FEniCS/dolfinx#2356)

Instead, flat std::vector or std::array structures will be used to store data, and
std::experimental::mdspan will be used to handle multidimensional data (https://en.cppreference.com/w/cpp/container/mdspan).

The Python-interface should not be affected by this PR.

[sonarcloud]

SonarCloud Quality Gate failed.   

0 Bugs
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0 Security Hotspots
12 Code Smells

0.0% Coverage
5.4% Duplication