Add dimension-templated constrained curve wrapper

[illionj]

First of all, thank you for this library. It has been very helpful in my work.

I ran into a small issue in my own use case: I mostly work with 2D curves, and
the current std::vector<fpPoint> representation means that each point is itself
a dynamically sized vector. In my path-smoothing code, this adds some extra
overhead and makes the call sites slightly less direct.

This PR adds fpConstrainedCurveTemplate<N>, an alternative fixed-dimension
wrapper around fitpack_constrained_curve_c.

The main change is that each point is represented as:

std::array<FP_REAL, N>

instead of a dynamically sized std::vector.

This is useful for common fixed-dimension cases such as 2D and 3D curves, while
keeping the existing API unchanged.

Example:

using Curve2D = fpConstrainedCurveTemplate<2>;
using Point2D = Curve2D::Point;
using Points2D = Curve2D::Points;

Points2D pts = {
    {0.0, 0.0},
    {1.0, 1.0},
    {2.0, 0.0},
};

Curve2D curve;
curve.new_fit(pts, 1e-4, 5);

Point2D p = curve.eval(0.5);
Point2D dp = curve.ddu(0.5, 1);

Changes

• Adds include/fpConstrainedCurveTemplate.hpp
• Exposes it from include/fitpack.hpp
• Uses std::array<FP_REAL, N> for points
• Keeps the existing fpConstrainedCurve API unchanged