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.

[jagerber48]

These floats are too large to work with. Prior to this PR the large float value could be saved into the AffineScalarFunc object and displayed, but any arithmetic with such large values would result in an OverflowError. In the new framework even displaying a UFloat the first time requires a calculation, even if it's as simple as sqrt(x**2). So in the new framework the OverflowError appears immediately.