Implement XTensor support in core.

[1uc]

Adds support for xt::xtensor, both row and column major. We're missing xt::xarray and views. Views fail because they claim to have runtime variable rank.

[codecov-commenter]

Codecov Report

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

Project coverage is 86.81%. Comparing base (ce46f86) to head (93670a6). Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #976      +/-   ##
==========================================
- Coverage   86.81%   86.81%   -0.01%     
==========================================
  Files         100      100              
  Lines        6083     6081       -2     
==========================================
- Hits         5281     5279       -2     
  Misses        802      802              

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[1uc]

@alkino The reason for not allowing non-trivial elements, is that this way we can use xt::adapt which also paves the way for implementing all other flavours of xt objects. With the same idea we will be able to take any xt object, and assign it to (read it from) memory we've wrapped with an xt::adapt. This idea works for row-/column-major and stridded/padded arrays.

It doesn't work for anything that contains non-trivial elements.

[1uc]

Closes #594.