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Re #20898

Re #20898

Re #20898

The unweighted cost function seems to work better with Levenberg-

Also additional unit tests, extended sphinx docs.

Re #20898

Re #20404

Re #20404

Re #20404

Re #20404

A concrete Jacobian implementation is not needed anymore in Mantid::API
namespace, so the move/rename of CurveFitting::Jacobian to
API::BasicJacobian is not needed anymore.

Re #20404

Use const static std::string for string constants instead of constexpr
char*.

Re #20404

Re #20404

Re #20404

Re #20404

Moved algorithm class private methods to a private namespace.

Re #20404

Also, extend unit tests.

Re #20404

Jacobian.h moved from CurveFitting to API and renamed to BasicJacobian
since a Jacobian implementation is needed by
CalculatePolynomialBackground.

Re #20404