Add row and col facade for 2 dim container - #1876

This pull request is related to #1189.
Row and col shortcuts for two-dimensional containers make the code more readable.

The implementation is straight forward. The row and col function can only be called on a two-dimensional container. If the functions are called on a container with other dimension an error occurs.
If the container has compile-constant dimensions, a static_assert will fail the build.
If the container has runtime-specific dimensions, a std::invalid_argument exception is thrown.

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  in both docs/source/changelog.rst and include/xtensor/xtensor_config.hpp according
  to the following rules:
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