Improve javadoc for iterative linear solvers with preconditioners

Preconditioning is the replacement of the linear system A * x = b with A * M^(-1) * y = b, followed by x = M^(-1) * y, where M approximates in some sense A. There is no consensus in the literature as to whether M of M^(-1) should be called the preconditioner.

In o.a.c.m3.linear, the Javadoc currently states that M is the preconditioner. However, following MATH-735, the solver must be passed M^(-1) (not M!) as a RealLinearOperator. This makes the whole Javadoc a bit obscure. It would be logical to call preconditioning the replacement of the initial system with A * M * y = b, where M approximates in some sense A^(-1) and will be called the preconditioner.

Such a change will make the javadoc more readable. However, it requires careful review of the existing Javadoc for the following classes

• PreconditionedIterativeLinearSolver,
• ConjugateGradient,
• SymmLQ,
• JacobiPreconditioner,

Also, in PreconditionedIterativeLinearSolver (and its concrete implementations), the parameter minv in solve() should be renamed m.