Refactor Cube class to use smart pointers

[maxlindbergh]

Developer Certificate of Origin
Version 1.1

This is in response to issue #1346 /Continue Modernizing Code’

This commit replaces raw pointers with std::unique_ptr for managing
the Mutex in the Cube class. The constructor and destructor have been updated accordingly, enhancing resource management and code safety.

Manual memory management with raw pointers is error-prone and
can lead to memory leaks or double deletion issues. By adopting
std::unique_ptr, we automatically manage the lifecycle of Mutex
objects, aligning with modern C++ standards and best practices.

• Ensures exception safety and automatic resource management.
• Reduces boilerplate code for manual memory management.
• Improves code readability and maintainability.

Affected Files:

• avogadro/core/cube.cpp

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[welcome]

Thanks for opening this pull request! Please check out our contributing guidelines and check for the automated tests.

[ghutchis]

I won't have time to do code review tonight, but thanks - I turned on the GH actions and I'll get back you tomorrow.

[ghutchis]

If you can't see the logs from the builds, let me know and I can send them to you or paste as a gist

[ghutchis]

The basic idea is good, but if you're going to use a unique_ptr you'll need to change m_lock in the header as well as the relevant methods and calling code.

Right now it doesn't compile, for example.