adding fixed point iterator class

written by Dylan Harries
comment closing namespace brace
make relative test function configurable via template parameter
adding test for fixed point iterator
adding note about slow convergence
factor out convergence tester
putting FPI convergence testes into separate namespace
add second fixed point iteration test with a perturbation term

This one converges quickly (5 iterations)!
adding second parabola example with problematic update function
adding support for user-defined addons

The addons must be placed in the addons/ directory.