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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[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.