Add missing default xbuffer_adaptor

[lavaux]

This pull request fixes the support for shared pointer by xtensor_adaptor/xbuffer_adaptor. The default constructor was declared but not implemented. A default implementation is sufficient. Also the move operators are brought

Checklist

• [x] The title and commit message(s) are descriptive.
• [x] Small commits made to fix your PR have been squashed to avoid history pollution.
• [x] Tests have been added for new features or bug fixes.
• [x] API of new functions and classes are documented.

Description

This small correction fix a link error when shared_ptr backing storage is used. It transforms an explicit constructor to a default constructor, otherwise the implementation is missing.

This pull request also adds unit tests for xbuffer_adaptor and xtensor_adaptor when a shared_ptr is used for backing storage. It tests the compilability and the semantic.