Fix #154: Add defaults for optional params and ConfigError for mandatory

[PranjalManhgaye]

This PR addresses #154 by making configuration handling more robust and explicit.

• Introduces a ConfigError exception and _get_required / _get_optional helpers to centralize config access and error reporting.
• Applies the documented defaults for optional configuration parameters (e.g., memory_usage_output_*, diagnostics, adaptivity options, snapshot options).
• Raises ConfigError with a clear path (e.g. simulation_params.micro_dt, coupling_params.precice_config_file_name) when mandatory parameters are missing or invalid, instead of generic KeyError / broad exceptions.
• Makes the diagnostics section optional in both coupling and snapshot modes while preserving existing behavior when it is present.
• Adds tests/unit/test_config_validation.py to cover missing mandatory parameters and default behavior for optional ones.

Testing

• Local: python -m unittest test_config_validation test_micro_manager.TestFunctioncalls.test_config -v

Checklist:

• I made sure that the CI passed before I ask for a review.
• I added a summary of the changes (compared to the last release) in the CHANGELOG.md.
• If necessary, I made changes to the documentation and/or added new content.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.

[PranjalManhgaye]

@IshaanDesai please let me know if you’d like any further changes or additional tests—i'm happy to adjust , thanks