Optimize tests

[aknierim]

Updates the entire test framework with unit tests that run faster than the old tests. Also, the new tests cover more edge cases than the previous tests.

Changes

• Removed tomli from dependencies and test requirements in pyproject.toml, added pytest-mock as a new test dependency
• Updated type hints across the codebase
• Added dataset_type as an to FITSWriter for consistency with other writer classes
• Updated linting configuration to exclude only examples from ruff checks, not tests
• Updated code coverage exclusion patterns
• Removed unused submodule pyvisgen.fits.data
• Updated __getitem__ method of ValidBaselineSubset dataclass to return ValidBaselinesSubset object, similar to Baselines dataclass
• pyvisgen.simulation.array.Array now has updated property names relative_pos and antenna_pairs. Removed calc_relative_pos method that would just get the relative_pos property