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- Removed dependencies on the 'ad' package since the core functionality didn't
really depend on it.
- Updated the umath sub-module to only utilize numpy for its mathematical
functions (i.e., sin, exp, abs, etc.).
- Added many constructor functions to make creating UncertainVariables easier,
like 'Normal', 'Uniform', 'Gamma', etc. All examples have been updated
accordingly to show the use of these constructors. The original constructor,
like 'uv(ss.norm(loc=2, scale=0.1))', is still functional.
- Updated the 'describe' method to allow the user to specify a printed
identifier as an input other than the 'tag' member.
- Added 'covariance_matrix' and 'correlation_matrix' utility functions.
- Renamed display text from 'UF(...)' and 'UV(...)' to just 'uv(...)'.
- Made the sample statistics permanent properties via the property decorator.