Code review

[slendidev]

• PolynomialSurface::fit can be made to take in a std::span instead of a vector reference.
• PolynomialSurface::getCoefficients can have it's implementation put in the header for better inlining.
• All doubles can be replaced with a templated type T, defaulted to double if you like, things like Eigen::MatrixXd can then be replaced with Eigen::Matrix<T, Dynamic, Dynamic>.
• In implementation, numTerms can be made consteval.
• Most functions can have their arguments be const, same as some variables in them like coeffs in fit.
• In PolynomialSurface::evaluate you can replace the for loop and result with this:

return std::inner_product(
  mono.begin(),
  mono.end(),
  coefficients_.begin(),
  0
);

• In the header, this:

    PolynomialSurface(int degree);  // Constructor: specify degree of polynomial

can be replaced with:

    PolynomialSurface(int degree_of_polynomial);

• Getters can be constexpr

[gbowne1]

@slendidev am about to start working on this improvement in the next 3-4 days.