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.
add interface for EWSB solvers
root finder and FPI implement the EWSB_solver interface
Minimizer implements the EWSB_solver interface
test all EWSB solvers
add solution getter to the EWSB_solver interface
check solution
make destructors of derived classes virtual
add more realistic EWSB example
make Fixed_point_iterator interface independent from GSL
remove template parameter from EWSB_solver