Unit tests

[rozyczko]

• Added a [project.optional-dependencies] section for test dependencies in pyproject.toml, specifying pytest and pytest-cov for running and measuring test coverage.
• Created tests/conftest.py to set up the Python path and provide a qcore_application pytest fixture for Qt-based tests.
• Added a docstring to tests/__init__.py to mark it as the test package for backend unit tests.

Unit Tests for Logic Modules:

• Added extensive tests for the following modules:
  • assemblies: Tests serialization, addition, type transitions, duplication, movement, and removal of assemblies.
  • layers: Tests serialization, creation of missing materials, property setters, duplication, movement, and removal of layers.
  • material: Tests serialization, addition, duplication, movement, removal, and property setters for materials.
  • experiments: Tests ordering, renaming, model association, removal, and index management for experiments.
  • calculators: Tests the availability and selection of calculator interfaces.
  • minimizers: Tests filtering, selection, and configuration of minimizers, including default behaviors.
  • fitting: Tests preparation, execution, error handling, and state transitions for fitting routines.
  • helpers: Tests message formatting and retrieval of original names from objects.