…r from the .hxx,

and correct several indexing/assignment issues:
- Replace unsafe pointer-based fill_n usage on ChangeValue with explicit per-column loops
  for TabWeights row 0 to ensure proper 2D array access.
- Fix coefficient offsets in ToCoefficients to account for array lower bounds (use ibegC and
  ibegJC), avoiding incorrect indexing into JacCoeff/Coefficients.
- Remove duplicate/unused include.

…last computed

error when the loop breaks early. This ensures MaxError is up-to-date when
NewDegree is set and returned.

Move the assignment of MaxError to occur after the tolerance/convergence check and potential break so MaxError always reflects the last computed Error value.

…strengthen tests

- PLib_JacobiPolynomial.cxx:
  - Remove unused <algorithm> and <cstring> includes.
  - Replace std::memcpy uses with explicit element-wise loops when filling TabWeights and TabMax to ensure safe 2D/1D array access.
  - Adjust ordering when populating row 0 in Weights to avoid overwriting database-supplied values.
  - Keep/ensure WorkDegree validation to fail early for invalid constraints.

- PLib_JacobiPolynomial_Test.cxx:
  - Expand ConstructorEdgeCases to validate minimum WorkDegree for each constraint level (C0, C1, C2) and keep a reasonable max-degree check.

These changes remove unsafe memory operations, fix potential out-of-bounds/aliasing issues, and make tests more explicit about valid degree/constraint combinations.

- Implement WorkDegree() and NivConstr() inline in PLib_JacobiPolynomial.hxx.
- Remove the #include of PLib_JacobiPolynomial.lxx from the header and delete the .lxx file.
- Remove PLib_JacobiPolynomial.lxx and PLib_JacobiPolynomial_Data.pxx entries from FILES.cmake.

Replace the manual ternary mapping (and inline throw for invalid orders) with
the centralized PLib::NivConstr helper, simplifying the constructor and
reusing existing validation logic.

- Replace magic integers/values with constexpr constants (NB gauss points, invalid value,
  max degree) and introduce constexpr lookup tables for DB pointers.
- Use std::array stack buffer in MaxError to avoid dynamic allocation and compute TabMax
  once into a local buffer.
- Replace ad-hoc pointer arithmetic and duplicated static tables with descriptive aDbPointer /
  aTrPointer variables and explicit 2D array copies for safe access.
- Rename function parameters and many local variables to consistent "the..." / "a..." / "an..."
  style for clarity; update header prototypes accordingly.
- Tighten validation branches, add braces, and normalize return/assignment logic in Points,
  Weights, MaxValue, MaxError, ReduceDegree, AverageError, ToCoefficients and D0123.
- Misc cleanup: move/clarify private section in header and remove hardcoded numeric literals.