Fix two bugs in ApprPolynomial4::Evaluate

[CodeReclaimers]

Two bugs in the Horner evaluation of ApprPolynomial4::Evaluate():

1. Line 249: writes to mYZCoefficient with stride mYDegree instead of mYDegreeP1, causing z-slice overlap.
2. Line 259: reads from mParameters instead of mYZCoefficient in the y-Horner loop, discarding the x-evaluation from stage 1.

Both bugs are masked when xDegree == 0. For xDegree >= 1 and yDegree >= 1, evaluation results are wrong.

Test (CMakeLists.txt)

cmake_minimum_required(VERSION 3.14)
project(test_issue_2_2_2_3 LANGUAGES CXX)
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)
set(GTE_ROOT "${CMAKE_CURRENT_SOURCE_DIR}/../../GTE")
add_executable(test_issue_2_2_2_3 test_issue_2_2_2_3.cpp)
target_include_directories(test_issue_2_2_2_3 PRIVATE "${GTE_ROOT}")

Test (test_issue_2_2_2_3.cpp)

#include <cmath>
#include <cstdlib>
#include <iostream>
#include <array>
#include <vector>
#include <Mathematics/ApprPolynomial4.h>

static bool test_trilinear_evaluation()
{
    using Real = double;
    auto poly = [](Real x, Real y, Real z) -> Real {
        return 1.0 + 2.0*x + 3.0*y + 4.0*z
             + 5.0*x*y + 6.0*x*z + 7.0*y*z + 8.0*x*y*z;
    };

    gte::ApprPolynomial4<Real> fitter(1, 1, 1);
    std::vector<std::array<Real, 4>> obs;
    for (Real x = 0.0; x <= 1.0; x += 0.25)
        for (Real y = 0.0; y <= 1.0; y += 0.25)
            for (Real z = 0.0; z <= 1.0; z += 0.25)
                obs.push_back({ x, y, z, poly(x, y, z) });

    fitter.Fit(obs.size(), obs.data());
    Real x = 0.37, y = 0.61, z = 0.83;
    return std::fabs(fitter.Evaluate(x, y, z) - poly(x, y, z)) < 1e-10;
}

static bool test_quadratic_xy_linear_z()
{
    using Real = double;
    auto poly = [](Real x, Real y, Real z) -> Real {
        return 1.0 + 2.0*x + 3.0*y + 4.0*z + 5.0*x*x + 6.0*y*y
             + 7.0*x*y + 8.0*x*z + 9.0*y*z + 10.0*x*x*y + 11.0*x*y*y
             + 12.0*x*x*z + 13.0*y*y*z + 14.0*x*y*z + 15.0*x*x*y*z
             + 16.0*x*y*y*z + 17.0*x*x*y*y + 18.0*x*x*y*y*z;
    };

    gte::ApprPolynomial4<Real> fitter(2, 2, 1);
    std::vector<std::array<Real, 4>> obs;
    for (Real x = 0.0; x <= 1.0; x += 0.1)
        for (Real y = 0.0; y <= 1.0; y += 0.1)
            for (Real z = 0.0; z <= 1.0; z += 0.1)
                obs.push_back({ x, y, z, poly(x, y, z) });

    fitter.Fit(obs.size(), obs.data());
    Real x = 0.42, y = 0.73, z = 0.55;
    return std::fabs(fitter.Evaluate(x, y, z) - poly(x, y, z)) < 1e-8;
}

int main()
{
    int bugs = (!test_trilinear_evaluation() ? 1 : 0)
             + (!test_quadratic_xy_linear_z() ? 1 : 0);
    return bugs > 0 ? EXIT_FAILURE : EXIT_SUCCESS;
}