Adds several improvements to DataAnalyzer, including

• Calculates image sizes in mm and their summaries (in additional to shapes and spacing)
• Saves the datastats.yaml file as 2 files, the summary and the statistics by case. The datastats.yaml will have only summaries, to be able to load it faster. Otherwise for a large datasets (with many labels) YAML loading takes over 2minutes.

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Integration tests passed locally by running ./runtests.sh -f -u --net --coverage.
• Quick tests passed locally by running ./runtests.sh --quick --unittests --disttests.
• In-line docstrings updated.
• Documentation updated, tested make html command in the docs/ folder.