Testing - Add unit tests for PLib functionality (#705)

Testing - Add unit tests for PLib functionality (#705)

- Introduced comprehensive unit tests for the Jacobi polynomial implementation in PLib, covering constructors, edge cases, Gauss integration points, weights, and basis function evaluations.
- Added tests for basic utility functions in PLib, including pole conversion and binomial coefficient calculations.
- Implemented checks for Hermite interpolation and polynomial evaluation with derivatives.
- Enhanced error handling and edge case testing for small and large coefficients.
- Initialised MaxError in PLib_DoubleJacobiPolynomial to ensure consistent behaviour during degree reduction.