Generalize SmoothWeightKernel to arbitrary degrees

[nmellado]

Switch to templated $(x^n-1)^m$

ponca/Ponca/src/Fitting/weightKernel.h

Lines 69 to 73 in 280968b

	PONCA_MULTIARCH inline Scalar f (const Scalar& _x) const { Scalar v = _x*_x - Scalar(1.); return v*v; }
	/*! \brief Defines the smooth first order weighting function \f$ \nabla w(x) = 4x(x^2-1) \f$ */
	PONCA_MULTIARCH inline Scalar df (const Scalar& _x) const { return Scalar(4.)*_x*(_x*_x-Scalar(1.)); }
	/*! \brief Defines the smooth second order weighting function \f$ \nabla^2 w(x) = 12x^2-4 \f$ */
	PONCA_MULTIARCH inline Scalar ddf(const Scalar& _x) const { return Scalar(12.)*_x*_x - Scalar(4.); }