Assert that XTensor objects are row-major.

[1uc]

Easy serializes XTensor objects by obtaining a pointer to the first element, and then using write_raw. Same for reading using read_raw.

Therefore, it only supports (a subset of) row-major arrays. This commit as a runtime check.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.38%. Comparing base (c1b3cde) to head (0c2fe58). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #989 +/- ## ======================================= Coverage 86.38% 86.38% ======================================= Files 92 92 Lines 5898 5898 ======================================= Hits 5095 5095 Misses 803 803 ```

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