RootFit: Implemented simple N-dim root finder.

Implemented simple version of Broyden's method.
Added all 1D test cases and a simple 2D test case from
http://www.polymath-software.com/library/problemlist.shtml for Broydens
root finder.
RootFinding: Add further test for 2D and 3D roots.
RootFinding: merge Broyden's method from 'taschna/master'
RootFinding: minor linear algebra and naming tweaks in Broyden method
Fit: multidim/vector version of linear combination fitting