Virtualize and Modularize DataSetLoader logic

[jshook]

This is the next step-wise change to streamline dataset loading and usage. It does the following:

• Virtualizes DataSetLoader, adds a central DataSets access point to simplify loading.
  • use Optional instead of checked IOExceptions
• Separates and modularizes HDF5, MFD loaders and consolidate helper methods.
• Organize dataset types into a package together.
• Update callers, with minor cleanups.
• Removes bench2d and its loader.

[github-actions]

Before you submit for review:

• Does your PR follow guidelines from CONTRIBUTIONS.md?
• Did you summarize what this PR does clearly and concisely?
• Did you include performance data for changes which may be performance impacting?
• Did you include useful docs for any user-facing changes or features?
• Did you include useful javadocs for developer oriented changes, explaining new concepts or key changes?
• Did you trigger and review regression testing results against the base branch via Run Bench Main?
• Did you adhere to the code formatting guidelines (TBD)
• Did you group your changes for easy review, providing meaningful descriptions for each commit?
• Did you ensure that all files contain the correct copyright header?

If you did not complete any of these, then please explain below.