Eliminate the Variable class

[jagerber48]

• Closes # (insert issue number)
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

This PR:

• Eliminates the Variable class and eliminates the AffineScalarFunc.derivatives property.
• The AffineScalarFunc class is renamed to UFloat with AffineScalarFunc left as a legacy name.
• The LinearCombo class is refactored into the UCombo class but the architectural role is similar: track the uncertainties of UFloat objects as a lazily expanded combination of objects with uncertainty
• The UAtom object is introduced as the fundamental element which captures uncertainty. Each UAtom models a random variable with zero mean and unity standard deviation/variance. Each UAtom has a uuid and all UAtom are mutually statistically independent. The expanded version of the UCombo maps dict[UAtom, float].

For more detail/discussion see #251 (comment)

Note that as of the time opening this PR the PR is still a work in progress. Not all elements in the bullet list above are implemented and a good handful of tests are still failing.