BornAgain

started with some easy refactoring

Halted after bbac161 (minor cleanup, and split of Fit/ into four directories with clear include hierarchy)

good progress in 008f21a;
base classes and utilities moved to Fit/ require second thought

rewrote title and description

priority set to high because rapid return to stability in directory structures is desired

I'll come back to this

Within #1466 and #1605 FitKernel was moved on board of Fit library. Now it is possible to find the minimum of arbitrary function

std::function<double(const std::vector<double> &)>

outside of FitSuite context. Function accepts parameters and returns the value. Result of minimization is a set of parameters, which gives a minimum value of the function.

To fit it, something like that should be done

// function whose minimum we are going to find
double TestFunctions::RosenBrock(const std::vector<double> &par)
{
    const double x = par[0];
    const double y = par[1];
    const double tmp1 = y-x*x;
    const double tmp2 = 1-x;
    return 100*tmp1*tmp1+tmp2*tmp2;
}

// code necessary to run fitting
    FitKernel kernel;
    kernel.setMinimizer("Minuit2", "Migrad");

    kernel.setObjectiveFunction(Rosenbrock);

    kernel.addFitParameter("par0", -1.2);
    kernel.addFitParameter("par1", -1.3);

    kernel.minimize();
    kernel.reportResults();
}

See also StandaloneFitTest.cpp from our functional test machinery.

In the future, it can be provided with python binding, the main actor FitKernel can be renamed, interfaces made even more convenient, and, as was planned, packaged as separate library. But I guess for this sprint, we have spend already lots of time on fitting. Also we have to let it lay a bit in peace, to may opinion, and to make our minds about future evolution.

Rejected, see #1823