Bug Fixes and Component-wise Force Simplification

[shenoynikhil]

Addressing the following issues

• Removing Nested Dictionary for Force Stats: Simplify component-wise force stats
• Force stats calculation: force_mean and force_component_mean was calculation incorrectly
• Stats before caching require iterating through StatisticsResults Object leads to error
• Loading stats with the right array format
• Interaction getitem had a bug which wasn't caught because of no tests. Adding tests in Refactor Interaction and Better Testing #71

Checklist:

• Was this PR discussed in a issue? It is recommended to first discuss a new feature into a GitHub issue before opening a PR.
• Add tests to cover the fixed bug(s) or the new introduced feature(s) (if appropriate).
• Update the API documentation is a new function is added or an existing one is deleted.

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[FNTwin]

PR was great! Tested a bit and added some code for the following:

• Pickling and loading StatsResult objects
• Simplified the get_statistics method
• Found a fixed a common reference bug on multiple dataset statistics
• Added files + new class for testing the API and prevent regression bugs
• Added tests