Overloaded the minEnclosingTriangle function such that there is an alternative function with the same name which does not require the output parameter ``area''.
Changed the sample source file minarea.cpp to use the overloaded version of the function.
Updated some comments in the min_enclosing_triangle.cpp source file.

Optimised one assert statement in the min_enclosing_triangle.cpp file. Added the minEnclosingTriangle functionality to the existing minarea.cpp sample file.

Added some assert statements to constrain the type of the input and output parameters. Convert the input set of points to vector<Point2f> before passing it to the findMinimumAreaEnclosingTriangle function.

1. The function receives a set of points as input instead of a convex polygon with more than three vertices
2. The convex hull P is computed inside the function
3.1. If the number of vertices of P is greater than three then the algorithm which was implemented before executes
3.2. Otherwise the vertices of the triangle are picked from the vertices of the given polygon and the area computed

- Changed the type of the minEnclosingTriangle function parameters by using proxy classes InputArray/OutputArray instead of std::vector<T>
- Adapted the source code to accommodate this change

- Added the source code for the function in the separate file min_enclosing_triangle.cpp

it conflicts with cv::detail

changed default parameters in CreateERFiltersNM{1,2} functions. They are not needed now in the sample example.

This allows to get rid of the last remaining Ptr<T>::addref calls.

Also changed the static functions to inline, to stop GCC complaining
about unused functions.

Now the features2d class wrappers use composition instead of inheritance.